Change your air requirements in what way? Because the requirements don't change, the supply does. And whales are a proof that an animal much bigger than a T-rex can in fact survive in current times oxygen level. Even though it lives in the fucking ocean.

I think it's a bit more complicated than that, blue whale are hyper efficient, they collapse their lungs to get all the oxygen they breath in into their blood stream, so they use the gas the breathe in many times more efficiently than a terrestrial mammal.

They also have hyper efficient heat retention, muscle use, everything. They are fully adapted for their milleaux, it's apples to oranges to compare them to a terrestrial bird/lizards that were mostly cold blooded and hollow boned.