system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do atmospheric oxygen levels have an effect on gigantism in vertebrates?

Someone recently suggested to me that high atmospheric oxygen levels were a factor that allowed dinosaurs to grow as large as they did.  I'd love to properly understand the matter and haven't been able to satisfactorily google an answer yet. (Answer doesn't need to be about dinosaurs, that's just how I stumbled upon the question.)
gpt
The main scientific theory agrees with your friend there, though is more commonly mentioned in relation to giant insects. In tests they found dragon flies grew much bigger in high oxygen environments.