Summary Cell size correlates with most traits among phytoplankton species. Theory predicts that larger cells should show poorer photosynthetic performance, perhaps due to reduced intracellular self‐shading (i.e. package effect). Yet current theory relies heavily on interspecific correlational approaches and causal relationships between size and photosynthetic machinery have remained untested. As a more direct test, we applied 250 generations of artificial selection ( c . 20 months) to evolve the green microalga Dunaliella teriolecta (Chlorophyta) toward different mean cell sizes, while monitoring all major photosynthetic parameters. Evolving larger sizes (> 1500% difference in volume) resulted in reduced oxygen production per chlorophyll molecule – as predicted by the package effect. However, large‐evolved cells showed substantially higher rates of oxygen production – a finding unanticipated by current theory. In addition, volume‐specific photosynthetic pigments increased with size (Chl a + b ), while photo‐protectant pigments decreased (β‐carotene). Finally, larger cells displayed higher growth performances and F v / F m , steeper slopes of rapid light curves (α) and smaller light‐harvesting antennae (σ PSII ) with higher connectivity (ρ). Overall, evolving a common ancestor into different sizes showed that the photosynthetic characteristics of a species coevolves with cell volume. Moreover, our experiment revealed a trade‐off between chlorophyll‐specific (decreasing with size) and volume‐specific (increasing with size) oxygen production in a cell.