Plant density and size are key determinants of yield but have rarely been analyzed explicitly in the context of biodiversity–productivity relationships. Here, we derive equations to partition the net biodiversity effect, complementarity effect, and selection effect into additive components that reflect diversity-induced changes in the number of plants and per-plant biomass. Applications of the new method to empirical datasets revealed contrasting ways in which plant density and size regulate the yields in species mixtures. In a tree diversity experiment, the causes of overyielding transitioned with stand development from enhanced growths in tree sizes to reduced mortality by complementary use of canopy space. In an annual-plant experiment, overyielding was largely explained by selection effects associated with increased plant sizes of highly productive species. The results highlight that the new method allows us to resolve crucial, yet understudied, demographic links between biodiversity and productivity.