Big-4 Concentration — 70% and EY's 32% Slice

Seventy percent of Thailand's SET Mainboard is audited by a Big-4 firm. Thirty-two percent is audited by EY alone. The remaining thirty-two percent — the non-Big-4 cohort of 204 firms — is where 57.5% of all current Kill Switch firms sit.

Three numbers, three structural readings. EY's single-firm market share of 32% is unmatched in any major Asian audit market the framework covers. The Big-4 / non-Big-4 split runs along an almost perfect cap-band line: zero Large-cap firms use a non-Big-4 auditor. And the Kill Switch concentration in the non-Big-4 cohort runs at 1.82× the universe baseline rate — a gap that gained regulatory salience after April 2026, when SEC Thailand's enforcement response to the Stark Corporation collapse made audit-firm choice less of a soft preference and more of a structural floor.

This Note documents the cohort. Note 1 of this series footnoted the 70% Big-4 baseline as a structural-quality floor; Note 7 used the audit-firm row in cross-market comparison as a B-axis-adjacent signal. Note 5 takes both forward into the deep dive.

The auditor map

The framework's auditor distribution across the 641-firm SET Mainboard universe, using strict Big-4 affiliation:^[1]

Auditor	n	%	Single firm name
EY	205	32.0%	EY Office Limited
KPMG	114	17.8%	KPMG Phoomchai Audit Limited (+2 affiliate)
PwC	89	13.9%	PricewaterhouseCoopers ABAS Limited
Deloitte	29	4.5%	Deloitte Touche Tohmatsu Jaiyos Audit
Big-4 strict total	437	68.2%	—
Non-Big-4	204	31.8%	(multiple firms)

A definitional note: the framework's broader internal audit-firm flag would assign 11 additional firms to Big-4 affiliations through Thai-local network entities, raising the count to 448 (69.9%) — the rounding that produces the seventy-percent headline used throughout this Note. For the structural narrative, the strict 437-firm count is the figure that matches public Big-4 affiliation conventions and is independently verifiable from SEC Thailand's auditor registry. The 11-firm gap is annotation, not material to the cohort reading.

The non-Big-4 cohort itself is not a single tier. It distributes across a clear "Thai domestic Big 3" plus a long tail of mid-tier and boutique firms:

Rank	Auditor	n	Tier
1	Dharmniti Auditing	34	Thai domestic-leading
2	DIA International Audit	27	Thai domestic-leading
3	Karin Audit	21	Thai domestic-leading
4	ANS Audit	12	Thai domestic mid-tier
5	Grant Thornton	12	International network mid-tier
6	SP Audit	10	Thai domestic mid-tier

The Thai domestic Big 3 — Dharmniti, DIA International, Karin Audit — together cover 82 firms, 40.2% of the non-Big-4 cohort and 12.8% of the full SET Mainboard universe. Grant Thornton, the largest international-network mid-tier non-Big-4, covers 12 firms.

Cap band: where audit choice stops being a choice

The Big-4 / non-Big-4 split runs along a cap-band line so sharp it functions as a structural prerequisite rather than a market preference.

Within each cap band, auditor distribution:

Cap band	n	EY	KPMG	PwC	Deloitte	Non-Big-4
Large (≥ THB 100B)	28	25.0%	50.0%	17.9%	7.1%	0.0%
Mid (10–100B)	113	41.6%	27.4%	20.4%	2.7%	8.0%
Small (< 10B)	486	30.5%	13.8%	12.1%	4.5%	39.1%

Two readings stand out.

Zero Large-cap firms on SET Mainboard use a non-Big-4 auditor. The 28-firm Large-cap segment (≥ THB 100B) is entirely Big-4-audited. KPMG dominates Large-cap at 50% (14 of 28 firms), with EY second at 25% (7 firms), PwC at 17.9% (5 firms), and Deloitte at 7.1% (2 firms). For an issuer to reach Large-cap status on SET Mainboard, a Big-4 audit relationship is not a quality preference — it is a structural floor.

Non-Big-4 is overwhelmingly a Small-cap phenomenon. 190 of 204 non-Big-4 firms (93.1%) sit in the Small-cap segment; 9 in Mid-cap; zero in Large-cap. Within Small-cap itself, the Big-4 / non-Big-4 split is roughly 60/40 — the only cap band where the non-Big-4 cohort has meaningful presence.

The Mid-cap segment is in transition. EY leads Mid-cap at 41.6% — the segment where EY's broad-coverage strength is sharpest — and the non-Big-4 share drops to 8.0%, which is well below its Small-cap rate of 39.1% but materially above the Large-cap zero. Mid-cap firms either move toward Big-4 audit relationships as part of their growth trajectory, or they stay in the Small-cap orbit indefinitely. The cap band is also the audit-firm-tier band.

Kill Switch: the headline reading

The non-Big-4 cohort's Kill Switch profile is the structural finding the rest of this Note rests on.

Auditor	KS firms	% of 40 KS	KS rate within cohort	Multiple of baseline
Non-Big-4	23	57.5%	11.3%	1.82×
EY	7	17.5%	3.4%	0.55×
KPMG	4	10.0%	3.5%	0.57×
PwC	4	10.0%	4.5%	0.73×
Deloitte	2	5.0%	6.9%	1.11×

A non-Big-4 cohort that is 31.8% of the universe carries 57.5% of all current Kill Switch firms. The KS rate within the non-Big-4 cohort runs at 11.3% — roughly three times the EY and KPMG rates of 3.4% and 3.5%. The Big-4 cohorts themselves cluster tightly: EY, KPMG, PwC sit between 3.4% and 4.5%. Deloitte's smaller 29-firm cohort posts the highest Big-4 KS rate at 6.9%, consistent with its more selective coverage profile.

EY, the largest auditor on SET Mainboard with 205 firms, posts the lowest Big-4 KS rate at 3.4% — marginally below KPMG's 3.5%. The largest book in the Thai audit market is also the cleanest among Big-4 cohorts. The scale of the EY portfolio has not produced quality degradation at the cohort level.

Archetype: the EY cohort signature

Auditor cohort archetype distributions, against the universe baseline:

Auditor	n	Celestial	Hidden Gem	Chameleon	Poison Apple	KS
EY	205	32.2%	14.6%	47.3%	2.4%	3.4%
KPMG	114	19.3%	12.3%	58.8%	6.1%	3.5%
PwC	89	19.1%	12.4%	56.2%	7.9%	4.5%
Deloitte	29	17.2%	31.0%	41.4%	3.4%	6.9%
Non-Big-4	204	1.0%	77.0%	10.8%	0.0%	11.3%
Universe	641	17.5%	34.5%	38.7%	3.1%	6.2%

Three readings.

EY's cohort runs Celestial-skewed. EY firms are Celestial at 32.2%, against the universe baseline of 17.5% — a 14.7-percentage-point overweight. EY's 205-firm portfolio, the largest audit book in Thailand, is structurally tilted toward governance-strong firms by archetype proxy. KPMG and PwC cohorts, by contrast, are Chameleon-dominated at 58.8% and 56.2% — large-cap-mixed-signal book composition.

Deloitte's small cohort is Hidden-Gem-tilted. The 29-firm Deloitte book is 31.0% Hidden Gem against the universe 34.5% — closer to baseline than the other Big-4 firms, with selective coverage that includes a higher share of small-and-mid-cap firms with B-and-R-OK profiles but T < 70.

Non-Big-4 is structurally bimodal. The 204-firm non-Big-4 cohort is 77.0% Hidden Gem (against 34.5% baseline — a 42.5-percentage-point overweight) and 11.3% Kill Switch (against 6.2% baseline). It is almost zero Celestial (1.0%) and zero Poison Apple. The reading is clean: non-Big-4 audit is the natural home of small-cap quiet-conscience firms (Hidden Gem) AND of distressed firms (Kill Switch). Above the distress line, the non-Big-4 cohort is the Hidden Gem cohort. Below it, the non-Big-4 cohort is where the Kill Switch concentration lives.

The framework's reading is not that non-Big-4 audit causes Kill Switch outcomes. It is that the cohort selects for both ends of the distribution — small-cap quiet-conscience firms that don't need Big-4 fees and don't fail, and small-cap distressed firms that can't get Big-4 sign-off and do.

Industry: where each Big-4 firm specializes

Auditor share within each SET industry group:

Industry	n	EY	KPMG	PwC	Deloitte	Non-Big-4
Agro & Food	71	42.3%	28.2%	9.9%	4.2%	15.5%
Consumer Products	47	23.4%	17.0%	8.5%	4.3%	46.8%
Financials	71	31.0%	18.3%	19.7%	16.9%	14.1%
Industrials	101	24.8%	16.8%	12.9%	0.0%	45.5%
Property & Construction	114	33.3%	12.3%	8.8%	4.4%	41.2%
Resources	57	21.1%	19.3%	36.8%	1.8%	21.1%
Services	135	34.1%	17.8%	10.4%	3.0%	34.8%
Technology	45	46.7%	15.6%	13.3%	4.4%	20.0%

Specialization patterns separate the four Big-4 books.

EY dominates Technology (46.7%), Agro & Food (42.3%), Services (34.1%), and Property & Construction (33.3%) — broad coverage strength across multiple sectors. KPMG runs second-position presence in Agro & Food (28.2%) and concentrates within the banking subsector, where Bangkok Bank, KBank, and other Thai-incumbent banking lineages are anchored. PwC carries a Resources specialization at 36.8% — the highest single-firm-industry share in the entire matrix, reflecting PwC ABAS's long-standing coverage of PTT-cluster energy and petrochemical entities. Deloitte concentrates in Financials (16.9%) and runs minimal presence in Industrials (zero) and Property & Construction (4.4%).

The non-Big-4 cohort is over-represented in Consumer Products (46.8%), Industrials (45.5%), Property & Construction (41.2%), and Services (34.8%) — the small-cap-heavy industries where audit-fee competitiveness matters most. Where the non-Big-4 cohort is under-represented — Resources (21.1%), Financials (14.1%), Agro & Food (15.5%) — the underlying issuer base is large-cap or family-cluster-anchored, and Big-4 relationships dominate.

Stark Corporation: the April 2026 inflection

Audit-firm choice on SET Mainboard was, until April 2026, a soft governance preference. After April 2026, it became a structural signal.

The sequence is on the public record. In mid-2023, Stark Corporation's accounting fraud disclosed — approximately THB 100 billion in fabricated international cable orders unwound when a new auditor refused to sign off on the FY2022 financial statements.^[3] SEC Thailand and the Department of Special Investigation initiated criminal proceedings; the corporate entity entered the delisting process. In April 2026, SEC Thailand banned Stark's audit firm — exercise of an enforcement authority that had been on the books but rarely used.^[4] The auditor was non-Big-4. The specific firm name and ban scope are matters of SEC Thailand public enforcement record.

The structural reading after April 2026 is not difficult to assemble from the framework data. Zero Large-cap firms on SET Mainboard use a non-Big-4 auditor. The non-Big-4 cohort's Kill Switch rate at 11.3% is materially elevated against Big-4 cohorts at 3.4–6.9%. The April 2026 ban gave audit-firm concentration regulatory salience that was previously assumed but unenforced.

What the regulatory shift made visible is a pattern the framework had been reading all along: the non-Big-4 cohort carries 57.5% of all current Kill Switch firms. The deep treatment of the Stark sequence — pre-event reconstruction, framework signal, post-event regulatory response — sits in Case 1 of this series.

What audit infrastructure does and does not require

SET Corporate Governance Code 2017 plus SEC's mandatory 56-1 One Report disclosures cover audit infrastructure with less prescriptive force than Korea's regulatory environment.^[5]

Dimension	Korea	Thailand
Mandatory firm rotation	Required since 2018 amendment to 외부감사법 (every 6 years for designated audit firms)	Voluntary; partner rotation only (7-year cooling-off)
Audit-fee disclosure	Standardized fee-band reporting	Baht amounts with category breakdown, less standardized
Audit committee independence	KCC §542-11 mandatory full-independent for large firms	SET CG Code 2017 expects with comply-or-explain

Korea's 2018 mandatory firm-rotation reform was driven by a wave of chaebol-related audit failures that exposed the limits of partner-only rotation.^[6] Thailand has consulted on firm-level rotation periodically but has not imposed the equivalent mandate. Without a forced-rotation mechanism, the historical EY market position has compounded across audit cycles — and the result is the 32% single-firm dominance the production data shows today.

Cross-market: the Thailand-distinctive concentration

No other major Asian market the framework covers has a single audit firm at 32% of universe share. Korea's KOSPI Big-4 cohort runs above 80% in aggregate, but the four Big-4 firms split roughly 25–30% each — Samil PwC, EY Hanyoung, KPMG Samjong, Deloitte Anjin sit in the same competitive tier.^[7] Japan's Big-4 audit market is dominated by KPMG AZSA and Deloitte Tohmatsu but neither runs above 30% of TSE Prime universe individually. Singapore's audit market is more dispersed across Big-4 affiliates plus international-network mid-tier firms.

Thailand's audit concentration is high in absolute terms (Big-4 at 68.2% strict) — and uniquely concentrated around one firm at 32%. EY's market-leading position traces to early-2000s acquisitions and audit-market consolidation; the resulting EY Office Limited is the single named auditor entity for all 205 EY-cohort firms in the production data.^[2] There is no internal sub-entity differentiation — one Thai EY firm, one auditor name, 205 listed clients.

The Thai audit market is concentrated around one firm. Korea's is balanced across four. The structural difference is regulatory: Korea forces rotation; Thailand does not.

What the auditor reading shows

Three structural takeaways.

First, audit-firm tier in Thailand is a cap-band sorter more than a quality assessment. The 28 Large-cap firms are entirely Big-4. The 486 Small-cap firms split roughly 60/40 between Big-4 and non-Big-4. Audit-firm choice tracks size before it tracks anything else.

Second, the non-Big-4 cohort is structurally bimodal. Above the distress line, it is Hidden Gem at 77% — small-cap quiet-conscience firms with B-and-R-OK profiles whose disclosure-cost profile keeps them below Big-4 fee thresholds. Below the distress line, it is Kill Switch at 11.3% — almost twice the universe baseline. The cohort selects for both ends of the small-cap distribution.

Third, the Thai audit market is uniquely concentrated around one firm. EY at 32% has no equivalent in Korea, Japan, or Singapore. The concentration compounded over two decades of consolidation in a regulatory environment that did not impose firm-level rotation. After April 2026, the regulatory environment shifted — and audit-firm choice moved from soft preference to structural signal at the same time the framework's reading of the non-Big-4 cohort showed why that shift mattered.

The framework's contribution is not to evaluate audit firms. It is to read the cohort. The cohort reading shows that Big-4 audit on SET Mainboard is the structural quality floor, that the largest book in the market has not posted cohort-level quality degradation despite its scale, and that the non-Big-4 cohort carries a Kill Switch concentration that the regulatory response of April 2026 made impossible to ignore.

Auditor market share on the SET Mainboard. EY's 32% single-firm share is unmatched in any major Asian audit market the framework covers.

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The Apex G-Score framework currently covers 641 SET Mainboard listed companies as of the April 2026 production snapshot. Underlying data: FY2025 cross-section. Scoring under TBR v2.0 weights: Transparency 0.30, Balance of Power 0.30, Conflict-of-Interest Risk 0.40.