Corporate research and development constitutes one of the main sources of innovation. Recent research, however, discusses a decline in corporate research and its implications for technological progress. The contribution of this study is to model research & development (R&D) decisions in an R&D investment model that allows the analysis of firms’ engagement in research (R) as compared to development (D) activities. The model predicts higher investments in both activities for larger firms, but it also shows that research intensity, i.e. the R-share in R&D, declines with firm size. We test these propositions using data of R&D-active firms over the period from 2000 to 2015. Results from panel model estimations that account for unobserved heterogeneity across firms show that larger firms invest indeed more in both research and development whereas the relative focus on research decreases with firm size. In addition, the empirical results suggest that, since the returns to research decline with firm size, specialization maximizes productivity. We discuss policy implications based on these findings.