This study empirically analyzed that audit quality in the accounting audit market is not unilaterally determined by factors such as auditor size, but rather is determined by the interaction between auditors as suppliers of audit services and firms as demanders of those services. Since DeAngelo (1981), auditor size and related factors have been widely used as proxy variables for audit quality, and much research has examined whether these proxy variables adequately reflect actual differences in audit quality. In Korea as well, many direct and indirect studies have examined whether audit quality differences exist based on auditor reputation and size, but the results have been unsatisfactory. While most studies in the United States and other countries have found that Big4 audit quality is higher than that of other auditors, in Korea, the differences in audit quality have not been consistently demonstrated, and at times no significant differences were found. However, prior research has been conducted to verify the hypothesis that large accounting firms' audit quality is consistently superior regardless of client firm size, and this assumption—that large accounting firms' audit quality is superior to that of other auditors across all client firm sizes—appears to be too strong. Most prior studies on audit quality dichotomized auditors into Big4 and Non-Big4 and sought to demonstrate that large accounting firms' audit quality is consistently superior regardless of client firm size. However, this research methodology was adopted directly from overseas prior studies without considering the realities of the domestic audit market, and due to these methodological limitations, the differences in audit quality among auditors in the Korean audit market have not been properly identified. For example, in Korea, small and medium enterprises account for 65% of listed companies that demand audit services, and due to these market conditions, Big4's market share stands at 57%, which is lower than in the United States and other countries. Furthermore, various legal forms of auditors exist, and because there have been legal restrictions on client firm size based on legal form and size, each auditor has developed expertise in differentiated primary audit markets. Therefore, this study examines whether these conditions in the domestic audit market explain why prior research findings have not been consistently demonstrated, and investigates whether audit quality is affected by the interaction between auditors as suppliers and firms as demanders of audit services in the audit market. To this end, firms were classified into five tiers by asset size, and an empirical analysis was conducted to determine whether the auditor providing high-quality audit services changes according to firm size. The results are as follows. First, for audit teams (gamsaban), which have limited audit revenue proportions and organizational form constraints, client firms' discretionary accruals were significantly higher than those of accounting firms, indicating that audit teams' audit quality is lower than that of accounting firms. Second, for small and medium listed companies with assets under 50 billion won, the discretionary accruals of firms audited by Local-Big and Local-Small auditors were lower than those of firms audited by Big4, indicating that Local-Big and Local-Small auditors provide higher audit quality than Big4 for small and medium enterprises, which constitute their primary audit market. Third, it was confirmed that the client firms for which Big4 can provide higher-quality audit services than other accounting firms are large enterprises with assets of 500 billion won or more, representing their primary audit market. These findings suggest that in Korea's audit environment, where each auditor has a differentiated primary audit market, the optimal auditor capable of providing the best audit services differs depending on firm size, and explain why prior domestic studies have not consistently demonstrated the superiority of Big4 audit quality across all firm sizes.