Why are cells so small?

In this video, I use diffusion cubes to show why cells are so small. A small cell with a large surface area to volume ratio can efficiently move materials into the cell.

Diffusion is a type of passive transport that cells use in order to move materials like oxygen and carbon dioxide into and out of the cell.
A cell with a larger surface area to volume ratio is more efficient a moving material than a cell with a smaller ratio.

Here is my recipe for the diffusion blocks
gelatin
Ammonia
Phenolphthalein
If you soak the diffusion cubes in vinegar then the phenolphthalein will turn pink when it comes in contact with the vinegar which allows you to see the rate of diffusion.