He's not fat he's big boned.
It's 3 million years worth of evolution of being that one giant badass in the tribe that decieves the predator that it's easy big win over the other skellies, and then it grabs the saber toothed tiger by the neck and crushes it to death in a choke hold. That's why people grow to the size of full size sedans today, if you don't figure out how to duct-tape your mouth closed and stop eating.
https://www.youtube.com/watch?v=ax5r-rxpDns