Implementing corporate sustainability strategies requires knowledge and application of sustainability management tools. While much progress has been made in developing such tools in both small and medium-sized enterprises (SMEs) and large companies, the literature claims company size positively affects application. However, the role of knowledge as a mediating factor has not yet been investigated. Using the knowledge-based view as a theoretical underpinning, this paper draws on empirical survey data from SMEs and large companies in Germany. It analyzes how company size affects the degree of knowledge and application of sustainability management tools. Even though the results reaffirm that SMEs know and apply significantly less tools, company size does not influence the share of tools applied once they are known. Thus, knowledge is identified as a key difference between SMEs and large companies as well as an important mediator to promote sustainability management.