We explore firms' commitment to Corporate Social Responsibility (CSR). Using a unique dataset of 8,857 French firms collected through a survey conducted at the end of 2011 by the French National Institute of Statistics and Economic Studies (INSEE), we first construct 3 CSR pillar scores for each firm, based on a non-parametric Item Response Theory model known as Mokken Scale Analysis. CSR scores, along with responses to specific items of the 2011 INSEE survey, allow us to characterize firms implementing formal versus informal CSR. We then estimate simple probit models and count data models to show that, with regards to CSR commitment, size definitely matters, and that a significant share of firms stating that they are not actively committed to CSR, actually engage significantly in CSR, with no monotonic size effect. Cooperation with external actors such as NGOs mitigates the size effect in the likelihood of carrying out informal CSR, whereas the pressure of NGO campaigns against large companies mainly spurs the environmental score of smaller firms in the same sector.