The sustainable development challenge is increasingly being included in entrepreneurs’ agendas. Firms are considered responsible for social and environmental effects but are also considered as social actors that can effectively incorporate sustainability solutions into market transactions. The literature on corporate social responsibility (CSR) in small business has depicted these firms as less involved in sustainability management implementation owing to resource constraints and limited perception of the business case for sustainability. Further, studies on both small business and sustainable entrepreneurship have highlighted the pivotal role of entrepreneurs’ values in motivating a more sustainable way of conducting business while, large companies, driven by external pressures, are more focused on a strategic CSR approach than small firms. Starting from these premises, the paper aims to identify the main drivers or barriers of sustainability implementation and to verify any significant differences between small and large-sized companies in their approach to sustainability practices implementation. The study adopts a qualitative research method based on semi-structured interviews addressing 22 participants from Italian firms of different sizes selected for their social and environmental commitment. The findings of the study highlight the existence of some common features among small and large firms, in particular, regarding motivations, entrepreneur values, and business vision, contributing new perspectives to the sustainable entrepreneurship debate.