I like the insight that air breathing animals have stronger energy input systems than those with gills. Respiration in mammals is an energy system evolved for l

I like the insight that air breathing animals have stronger energy input systems than those with gills. Respiration in mammals is an energy system evolved for land, where the constraints involve 5 degrees of freedom and normal g. The ocean has 6 degrees of freedom and reduced g. So in the case that an organism's niche can be improved by evolving to be larger and they have an energy system which can support more flesh, doing so is easier in the ocean than on land. It's true for all ocean animals but it's especially true for mammals.