system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't Godzilla-esque monsters exist in nature ?

I understand that food and energy levels would be and issue, but is there anything else stopping gargantuan creatures evolving ?

Same for giant flying creatures, bigger versions of real animals, insects and sea creatures.

Thanks.
gpt
Gravity and oxygen are your limiting factors. Too big, and you can't form bones strong enough to support the weight, and you would need gigantic lungs to support the body mass required. You find larger animals in the ocean, such as blue whales, because gravity isn't a factor there. 

In the time of the dinosaurs, there was a much higher concentration of atmospheric oxygen, so it was easier to get the required oxygen.