On the other hand, fat guys are harder to hurt.
The fat is additional padding over bones, organs,
muscles, tendons, etc. Have you noticed it's
way more difficult to knock the wind out of fatter guys? Fewer targets are exposed as the usual ouch spots are protected. Knife penetrations won't go as deep, too.
If you do manage to kill a fat guy, you'll have more soap to sell to high-end cosmetics stores.
There's a guy at my gym that carries a bit of extra weight. We were working on ground and pound skills and I could throw full power body shots on him without causing little more than a grunt.
I guess it all depends on what you're training for, and what you're trying to accomplish.
And it goes back to the old coach/player argument. A withered old boxing trainer might not be able to do much against an average guy on the street, but he's a resource of knowledge on the sport of boxing and when it's passed to someone gifted with athletic ability, their game improves.
Personally, I feel anyone in a combat-sport related art that's of competition age should have a respectable level of fitness, regardless of body composition.
Other than that, it's a tough call.
While at a large tournament, I went out and had a few beers with some of the serious senior belts that I am aquainted with. One of them is pushing about 60, has a huge beer gut, and smokes like a chimey... he is also a 9th Dan. I have seen him spar, and I have trained with him... I would really hate to piss him off... he is fast, has brutal power, and lay some serious smack down. Thankfully he is a great guy!
I guess no one heard of Tuhon Tom Kier(now thats a big boy) from Sayoc Kali who broke "Top Dog"Eric Knaus's leg at a Dog Brother Gathering.Sayoc a "bs" art? Hmmmm....
"Come where the flavor is,come to KALI country"
What do fat people need self-defense skills for? They hit or sit on you, you're finished.