system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How come insects are so small?

I watched this series that said it has something to do with the amount of available oxygen while they're developing. If that's true then would it be possible to just pump an ant colony up with hella 02 and make like puppy sized ants?
gpt
Insects don't have lungs, they absorb oxygen directly through their skin. This limits how big they can get, because the amount of oxygen they need scales with volume but the amount they can get scales with area...as they get bigger, the ratio gets worse and eventually they literally can't breathe (there's an entire Doctor Who about this with a giant spider). 

Insects have evolved under this constraint, making them the size they are. If you suddenly give them 100% O2 then they can certainly breath better (just like us) but they don't suddenly grow bigger anymore than you eating 4000 calories a day makes you get taller or fertilizing your carrots a lot will make them 100' tall. In theory, over very long periods (thousands of years) they could evolve to be bigger if there was a survival advantage to doing so. But 100% O2 is also extremely toxic to most organisms long term, so it's not that simple either.