Well, I dunno about fat, but Ganondorf from the Wind Waker put on a few pounds after Hyrule flooded.
But I think that the fattest boss is Jahalla from the Wind Waker, maybe just a bit bigger than the Deku Toad from Twilight Princess.
SS moblins are also fat. Really fat. But even though this topic is 13 years old I also agree wirh Toad or Jahalla. But the latter was more many poes combined