Cells are so little, so they can maximize their ratio of surface area to volume.
Explanation:
Smaller cells have a higher ratio which allows more molecules and ions to move across the cell membrane per unit of cytoplasmic volume. Cells are so small because they need to be able to get the nutrients in and the waste out quickly.
This is also called light. They can move through empty spaces because do not need a medium and they are the fastest because no matter how far they are they do not get smaller.
