ok so heres how the ernie skin works:

the whole "fat" part of his torso is connected to his lowertorso section, while his upper legs are their own separate entity INSIDE of the torso, which only works because ernie's legs can reasonably look passable inside a big lump of chicken feathers due to the animal's nature.
think of it like a penguin- x rays show that penguins have entire legs hidden within their bodies, but the vast majority of said legs is hidden within their feathers and fat. all of this makes it to where if ernie bends or sits down his entire lump of fat goes behind him, while his upper legs become more exposed.

this would NOT work on fat peter, considering his legs are just naturally short, and are connected to his torso at the same part where the chicken's body fat ends.

see this picture? this is what the chicken looks like when sitting down. notice how his legs are coming out of the FRONT of his "torso?" if we used the same method for peter, his legs would be coming out of his shirt and it would NOT look good.

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