Why are cells small?

Most of the process of moving nutrients into and out of a cell is accomplished by diffusion or osmosis. A cell is placed next to a nutrient-rich liquid and the nutrients are passed through the membrane surface. If the surface area is small, there is a not a lot of transfer. In order to maximize the process of osmosis, it is beneficial to have a large surface area for each cell. It is also beneficial to have a small volume for the cell. In this way, the nutrients will be more concentrated.