Here's why:

* Diffusion and Osmosis: These are passive transport processes where substances move from an area of high concentration to an area of low concentration.

* Cell Size: As a cell grows larger, its surface area to volume ratio decreases. This means that the cell has less surface area relative to its volume.

* Effectiveness: A smaller surface area to volume ratio makes diffusion and osmosis less effective. The cell's internal structures are further away from the cell membrane, making it harder for nutrients to enter and waste products to exit.

* Staying Small: To maintain efficient transport, cells need to keep their surface area to volume ratio high. This is why cells stay relatively small.

Therefore, as cell size goes decreases, the effectiveness of diffusion and osmosis. This drives the cells to stay small.