This study examines the impact of platform innovation on the efficiency of firms located in different clusters. The sample contains data for 1,257 firms from 2005 to 2015 in Beijing’s Zhongguancun Science Park, which is known as China’s ‘Silicon Valley’. Using the DEA model to gauge the efficiency of each firm over time, we found that firm size has a significantly negative correlation with knowledge production efficiency, a significantly positive correlation with its knowledge commercialisation efficiency, and an inverted-U shaped relationship with overall efficiency. We then explored the mediating effect of firm size on the relationship between the characteristics of clusters and firm efficiency, before concluding with both managerial and academic implications.