It wasn’t just extra oxygen. The key that is hardly ever mentioned is that the atmospheric pressure must have been much higher as well. High enough, in fact, that the atmosphere’s pressure allowed extremely large creatures to live on land, due to buoyancy, like they still do below the oceans.
If you just had more oxygen in the atmosphere, but the same pressure, then creatures would not have grown much larger than they do today. They would have reached their full size faster, but they couldn’t grow much larger.
Sorry, but if you even DOUBLED the air density - i.e., the number of molecules of N2 and O2 per ccm - that would only increase it from about 1.4 gm/liter to 2.8 gm/liter. An animal weighing 100 kg and having a density of about 1 gm/ccm (like a human) would experience a reduction in perceived weight (due to the added buoyancy) of only 140 grams. That is negligible.
If you increased the air density by 10X, that STILL only reduces the apparent weight of a 100-kg human by 1.4 kg - or by 1.4%.
Regards,