Following the total quality management (TQM) philosophy and the knowledge management (KM) approach, this contribution aims to study the influence of process methodology and partner management on KM, as well as the relationships between this variable and key business results. We also analyse the moderating role of organisational size in these previous relationships. The hypotheses proposed in our research model are tested on a sample of 225 Spanish companies with experience in TQM through evaluations using the EFQM Excellence Model. The partial least squares structural equation modelling (PLS-SEM) approach was used to test the research model. In order to assess the moderating effects of organisational size, we adopt a multi-group approach using two subsamples with large firms and small and medium-sized enterprises (SMEs). Our findings indicate that the use of process methodology and the involvement of partners are key factors for KM to have a significant impact on the key results of the business, both strategic and operational. Moreover, the organisational size is determinant when analysing the effect of process methodology and partner management on KM. In this sense, process methodology has a greater effect on KM in the SMEs. On the contrary, the relationship between partner management and KM is more intense in large firms. Finally, it is noted that KM can be effective and can improve the key business results independently of the size of the organisation.