As the size of a cell increases, its ability to facilitate diffusion across its cell membrane decreases. This is because the internal volume of a growing cell, or any three-dimensional enclosed structure, increases by a greater proportion than its external surface area. If a cell were to grow in size past a certain point, its outer surface, or plasma membrane, would no longer be able to keep up with the greater demands required of the diffusion process by its enlarged interior.
The cytoplasm and organelles within a cell obtain nutrients and eliminate wastes though the cell's plasma membrane. A cell would, however, begin to starve if it increased in size past the point where its plasma membrane possessed enough surface area to diffuse the required amounts of cellular nutrition. The lowered diffusion rate and the accompanying decrease in the speed of waste elimination would also cause the cell to be poisoned by a build-up of toxic substances.
Because of the need to maintain the proper ratio between interior volume and external surface area, cells will reproduce rather than grow past a certain point. This accounts for why larger multi-cellular organisms do not have larger cells; instead, they have a greater number of cells. Smaller cells are better suited to enable diffusion because smaller objects have a higher ratio between surface area and interior volume.