Naw, I disagree... if you include all the armour he more-than-likely has, and the fact he's pushing out his chest then it's perfectly feesable he isn't fat at all. And don't get me started on the robe... he hides swords up his sleeves, man! You could hide a whole army of moblins under there and you'd never notice.
Also, I'd like to bring to people's attention this little gem...
Some concept art of Ganondorf before they decided to give him that huge robe... now, I know characters can change a little before the final game, but he sure doesn't look that fat to me there. Bulky, yes. Fat? No. (Apart from in the top left drawing... but that's just how he's stood XD) And that's more than likely what he still looks like under such a baggy robe.
EDIT: VanitasXII makes a brilliant point as well... if he was just plain fat, how on earth is he so agile? He needs all that muscle to be able to propel himself so far off the ground when he jumps XDD