Body size of life on Earth spans many orders of magnitude, and with it scales the energetic requirements of organisms.  Thus, changes in environmental energy should impact community body-size distributions in predictable ways by reshaping ecological and niche dynamics. We examine how carbon, oxygen, and temperature, three energetic drivers, impact community size-based assembly in deep-sea bivalves. We demonstrate that body-size distributions are influenced by multiple energetic constraints.  Relaxation in these constraints leads to an expansion of body-size niche space through the addition of novel large size classes, increasing the standard deviation and mean of the body-size distribution. With continued Anthropogenic increases in temperature and reductions in carbon availability and oxygen in most ocean basins, our results point to possible radical shifts in invertebrate body size with the potential to impact ecosystem function.