Here's why:

* Surface area to volume ratio: As a cell grows larger, its volume increases much faster than its surface area. This means that a larger cell has less surface area relative to its volume, making it harder for the cell to exchange nutrients and waste with its surroundings.

* Diffusion limitations: Cells rely on diffusion to transport substances across their membranes. Diffusion is much more efficient over short distances, so smaller cells are better equipped to meet their needs.

* Efficient function: Smaller cells are able to maintain a high surface area to volume ratio, allowing for faster and more efficient nutrient uptake, waste removal, and communication with other cells.

However, it's important to note that "small" is relative. Animal cells can vary greatly in size, from the microscopic (like red blood cells) to the relatively large (like nerve cells). But compared to other types of cells (like plant cells or bacterial cells), animal cells are generally on the smaller side.