Biosynthetic capacity, the nature of cell division machinery and the fact that cells grouped together form various organs and structures are factors thought to influence and limit cell size in animals. The full extent of the processes that regulate cell size are not fully understood.
Biosynthetic capacity refers to the fact that nutrient exchanges vital to the cell's survival depend on the surface area of the cell. Cell division or mitosis requires certain structures such as the mitotic spindle and microtubules. There is an optimal size where the cell is large enough to accommodate what is necessary but not too large to effectively cleave during cytokinesis.
Cells are packed together to form specific organs at a density that optimizes biochemical reactions. This factor also limits cell size.