This paper reports the analyses of the social and environmental disclosures of listed South African mining companies and compares the disclosures of larger companies with those of smaller companies using several different categories of comparison. The prior literature suggests that larger companies almost invariably disclose more social and environmental information due to their greater visibility. The expected differences are found in social disclosures, but not in environmental disclosures. An institutional theory framework is used to explain these unexpected environmental disclosure findings. Specifically, the field of corporate environmental disclosures among mining companies may have reached a level of maturity and professionalization causing disclosures to become similar through a process of isomorphism. These similarities have now reached the stage where small companies disclose the same amount of environmental information, in the same general format, as large companies. While all three types of isomorphism (mimetic, coercive, normative) are probably still at work, in theory normative isomorphism, usually driven by professionalization, becomes more prominent when a field reaches maturity.