Natural selection on photosynthetic performance is a primary factor determining leaf phenotypes. The complex CO$2$ diffusion path from substomatal cavities to the chloroplasts – the mesophyll conductance ($g ext{m}$) – limits photosynthetic rate in many species and hence shapes variation in leaf morphology and anatomy. Among sclerophyllous and succulent taxa, structural investment in leaves, measured as the leaf dry mass per area (LMA), has been implicated in decreased $g_ ext{m}$. However, in herbaceous taxa with high $g_ ext{m}$, it is less certain how LMA impacts CO$2$ diffusion and whether it significantly affects photosynthetic performance. We addressed these questions in the context of understanding the ecophysiological significance of leaf trait variation in wild tomatoes, a closely related group of herbaceous perennials. Although $g ext{m}$ was high in wild tomatoes, variation in $g_ ext{m}$ significantly affected photosynthesis. Even in these tender- leaved herbaceous species, greater LMA led to reduced $g_ ext{m}$. This relationship between $g_ ext{m}$ and LMA is partially mediated by cell packing and leaf thickness, although amphistomy (equal distribution of stomata on both sides of the leaf) mitigates the effect of leaf thickness. Understanding the costs of increased LMA will inform future work on the adaptive significance of leaf trait variation across ecological gradients in wild tomatoes and other systems.