Maintaining high rates of photosynthesis in leaves requires efficient movement of CO₂ from the atmosphere to the mesophyll cells inside the leaf where CO₂ is converted into sugar. CO₂ diffusion inside the leaf depends directly on the structure of the mesophyll cells and their surrounding airspace, which have been difficult to characterize because of their inherently three-dimensional organization. Yet faster CO₂ diffusion inside the leaf was probably critical in elevating rates of photosynthesis that occurred among angiosperm lineages. Here we characterize the three-dimensional surface area of the leaf mesophyll across vascular plants. We show that genome size determines the sizes and packing densities of cells in all leaf tissues and that smaller cells enable more mesophyll surface area to be packed into the leaf volume, facilitating higher CO₂ diffusion. Measurements and modelling revealed that the spongy mesophyll layer better facilitates gaseous phase diffusion while the palisade mesophyll layer better facilitates liquid-phase diffusion. Our results demonstrate that genome downsizing among the angiosperms was critical to restructuring the entire pathway of CO₂ diffusion into and through the leaf, maintaining high rates of CO₂ supply to the leaf mesophyll despite declining atmospheric CO₂ levels during the Cretaceous.