Why can't bacteria grow bigger than humans?

Answer 1

Bacteria are limited in size due to their energy needs. For a bacteria to grow very large, it would need to increase the amount of "pieces" inside to make energy (for example, ribosomes). There is a point where the bacteria would need more ribosomes to produce energy than could actually fit inside it and so it can't grow past a certain size. There are also advantages to being small, it is more efficient for transport inside and out side the bacteria, and since bacteria are unicellular they can't have the diverse cell types you need to form a tissue or perform many different functions.