I train with weights and big gloves all the time, that's not the same as having thirty pounds of gut hanging off your belly though. I realize subcutaneous fat distributes itself all around your body but it never looked like he had a ton of fat on his arms to me...
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.