Cells are limited in size by their surface area and volume. A cell does not possess a mouth, lungs and digestive system. Every process has to happen directly or indirectly through the cell membrane. The relationship between the volume of cell contents and the surface area of the cell membrane becomes the limiting factor in how big a cell gets to be. If it gets too big it may starve due to lack of incoming nutrients, or poison itself because it can not get rid of waste rapidly enough.