Tetrapods dominate because they have bones, instead of exoskeleton which needs to be replaced and leaves them vulnerable and consumes a lot of energy 

They aren't ginormous because of this! This makes more sense than oxygen [[How bugs breathe]]

#rewrite This is a fleeting note containing a raw insight. It lacks a formal structure, a clear title, and supporting evidence. It needs to be expanded into a permanent note that details the metabolic costs of molting and the structural advantages of endoskeletons compared to the oxygen-limitation hypothesis.