i can't speak for all guys, but most of my guy friends who are in longterm relationships or have gotten married have put on a little weight, including me. i think that's just natural. guys are no longer worried about attracting a mate so appearance is not as high on the list. my guy friends who are still single and dating still worry about their physical appearance, including my brother. if you don't buy that argument, then there's always the point that as you get older your metabolism slows down and it becomes easier to put on weight.
therefore, one could make the argument that tom wasn't worried about his appearance anymore b/c he had settled down with b'elanna and was no longer worried about being a "lady's man."