From the fossil records we can tell that insects were much larger many million years ago. Today insects are small, while some of them are tiny.
We till do not really know why insects are that small today. The theory of "heavy exoskeleton" does not really answer this question, since Meganeura, a 300million years old bug was larger than a human baby.
Another theory suggests that the limited oxygen in the air caused the insects to grow smaller. Insects have a very different breathing system then we do. They do not inhale air into lungs, instead the air come inside the body of the insect through very small holes across its body. These holes become smaller and smaller down to the size of a nanometer. Big insects should probably have very long air tunnels and would require more oxygen to support it. 300 million years ago, there were indeed much more oxygen in the air than today which perfectly supports this theory.