A punch doesn't come only from the arm. It's the whole body that's behind a punch. Try it with a weight vest.
I'm not saying his is the ideal body type, but he certainly has more punching power with that additional mass than without it.
That's a good point but I'd wonder whether he ends up giving up power after losing weight would depend on how he's trained to get that weight off. If he's gained some quickness, improved his cardio and has more explosive power as a result of a better diet and conditioning program then I'd have to think that those attributes would contribute to his power moreso than that big ass spare tyre around his waist.