["Business Strategy and the Environment, EarlyView. ", "\nABSTRACT\nFocusing on firm‐size heterogeneity, this study examines how institutional reform reshapes the effects of environmental, social, and governance (ESG) disclosure on firm value. Using 2019–2023 panel data on 1427 Japanese listed firms (before and after the 2022 Tokyo Stock Exchange reorganization and Corporate Governance Code revision), this analysis investigates how the valuation consequences of ESG disclosure vary across institutional conditions. Correlated random‐effects (CRE) and CRE–two‐stage least squares models using industry‐average ESG disclosure as an instrument show size‐based differences. Before the reforms, ESG disclosure was mainly associated with firm value among large firms, while mid‐sized enterprises tended to experience negative valuation effects. After the reforms, stronger coercive pressures and standardized disclosure requirements weakened valuation benefits for most large firms, with positive effects tending to be confined to only very large firms. Overall, quasi‐mandatory disclosure improves transparency but reduces strategic differentiation, highlighting the need for more proportionate ESG disclosure frameworks.\n"]