Why are cells small?

Cells are constrained to small sizes primarily by two physical limitations: the surface area-to-volume ratio, which determines how efficiently a cell's membrane can deliver nutrients and remove waste, and diffusion rates, which govern how quickly molecules encounter each other for essential cellular processes. While cell size ultimately depends on function—sperm are tiny to conserve energy while oocytes are large to store nutrients for embryonic development—physics fundamentally limits how large any cell can grow before metabolism slows and molecular collisions become too infrequent to sustain life.