What may account for the small size of the cells?

For all the chemical reactions which are part of metabolic processes to go smoothly, cells require quick absorption of molecules through the membrane at the same time they should be able to dispose of molecules quickly. The smaller the size of the cell large is the surface area to volume ratio. This large ratio of surface area to volume makes it easier for cells to transfer molecules and ions in and out of themselves. This small size enables them to allow nutrient molecules to get in and waste material to get out of the cells rapidly, making the cell more efficient. Smaller cells can perform such metabolic tasks much more quickly and easily than the cells which are comparatively bigger in size. This happens because, as discussed earlier for bigger cells, the surface area to volume ratio becomes smaller making it difficult to transport molecules and ions in and out of them. Thus, to make the process of transport easier and perform metabolic processes more efficiently, cells are generally small in size. The size of the cell and the surface area to volume ratio is inversely proportional to each other. The smaller the size the better and faster is the transport of molecules and ions. And as the cell becomes bigger it becomes difficult for molecules and ions to pass through it.