The survival of cells and the subsequent organisms that they form is dependent upon their ability to exchange gases, nutrients, ions and waste with their environment. The biggest limiting factor in a cell's ability to complete these processes is size. There are three main size-related factors that contribute to a cell's small size allowing them to maximize efficiency and ensure survival.
Diffusion is the process by which cells move nutrients and waste into and out of the cell. There are several different types of the diffusion that cells utilize including those that require energy and transporters. An increase in cell size would decrease the rate of diffusion, which could result in a build up of waste inside the cell as well as nutrient deficiency.
DNA contains the genetic blueprints for protein synthesis. All organelles of the cell require proteins to function; therefore, an increase in cell size would require an increase in DNA and protein synthesis to accommodate the extra organelles. Smaller cells require less protein and are more manageable than a larger cell.
Surface Area to Volume Ratio
The volume of the cell increases at a much faster rate than the surface area of the cell. The surface area represents the plasma membrane, which is the site of diffusion. A cell that is too large in volume does not have enough plasma membrane to effectively move particles in and out of the cell.
These limiting factors are the reason the cells divide instead of grow larger. Through cellular division, cells are able to maintain their optimal small size.