Small and medium-sized firms form 90% of the worldwide population of businesses. However, it has been argued that given their smaller scale of operations, resource access constraints and lower visibility, smaller firms are less likely to participate in Corporate Social Responsibility (CSR) initiatives. This article examines the different economic motivations of firms with varying combinations of visibility, resource access and scale of operations. Arguments are presented to propose that in terms of visibility, resource access and operating scale, very small and very large firms are equally motivated to participate in CSR. However, the motivational bases for CSR participation are likely to be different. Medium-sized firms are the least motivated. This suggests a U-shaped relationship between firm size and CSR participation. This study contributes towards resolution of the long-standing debate on the effects of firm size on CSR participation, and highlights the importance of considering configurations of firm characteristics in the study of CSR outcomes. In conclusion, cautions are raised against the broad categorization of firms, without adequate attention to the underlying dimensions of such categorizations.