Studies on the size‐scaling of phytoplankton growth rate are usually based on temperature‐corrected growth rates or experiments performed at a fixed temperature, but the effects of differing thermal adaptation of small and large species have not been considered. We use an extensive dataset of phytoplankton growth rate responses to temperature and cell size to show that the unimodal size‐scaling of phytoplankton growth depends strongly on temperature, and is not significant at high temperatures where the most common picophytoplankton species grow at their optimum. Furthermore, we show that the unimodality results from the different growth rate scaling of picophytoplankton, which differs phylogenetically from larger phytoplankton taxa. Using ribosomal RNA sequences we recalculated the size‐scaling allometry with Phylogenetic Generalized Least Squares regression. After phylogenetic correction, the unimodal relationship is not significant at any temperature, suggesting that the observed curvature reflects the evolutionary adaptation of picophytoplankton to the warm conditions usually encountered in oligotrophic environments.