This study examines the impact of International Financial Reporting Standards (IFRS) adoption on financial reporting quality in Nigerian quoted manufacturing companies, focusing on variations across time (2020–2024) and firm size categories. Adopting a descriptive survey research design with a quantitative approach, a purposive sample of ten manufacturing companies listed on the Nigerian Exchange Group (NGX) was selected based on full IFRS compliance and availability of complete audited financial statements from 2020 to 2024. Secondary data from annual reports were analyzed using descriptive statistics and Analysis of Variance (ANOVA), with Return on Assets (ROA) as a proxy for financial reporting quality. The results reveal no statistically significant differences in mean ROA across the years 20202024 (F(4,26) = 0.5854, p = 0.8043), indicating that IFRS adoption did not produce measurable year-to-year improvements in reporting quality during the review period. However, significant differences were found across firm size categories (F(2,27) = 13.1752, p < 0.001), with large firms demonstrating substantially higher mean ROA (0.1378) compared to medium (0.0485) and small (0.0212) firms, suggesting that larger firms benefit more from IFRS adoption due to superior resources and technical capacity. The findings imply that IFRS adoption alone is insufficient to guarantee improved financial reporting quality in developing economies like Nigeria; regulators must strengthen enforcement mechanisms and provide targeted capacity-building initiatives for smaller firms, while audit firms and corporate boards should prioritize governance structures that support rigorous IFRS implementation. This study contributes to the limited empirical literature on post-IFRS adoption outcomes in Nigerian manufacturing firms by highlighting the moderating role of firm size, offering critical insights for policymakers seeking to bridge the compliance gap between large and small firms in emerging markets.