What aspect of a fighter's biology makes them move up?

Hagler fought in an era with less weight classes. The next one up was 15lbs north, at i believe 175. Hagler would be too small to compete in this division imo. Hagler stayed in the same weight class his whole life, because he found the weight he was best at, and it was natural for him. He boxed at 165 in the amateurs, fighting against naturally bigger men. He stayed in shape his whole career, (not balooning up to 200lbs like a jr welter weight ricky hatton inbetween bouts). So at his natural weight he was able to dominate. Fighters switch weights too much these days when they should find their best fighting weight, and should stay there. Unless they have a legitimate reason to move up, like getting beat by the best in the division and seeking new opportunities elsewhere. (ex. adamek vs dawson).

w/ age your metabolism slows down a bit so it's hard to lose weight. I remember I eat twice as much 10yrs ago than today and still have rock hard abs w/o even going to the gym. Thanks for this, this was the kind of answer I was looking for

How are some fighters able to stay in one weight class for their entire career like Marvin Hagler, while others jump from weight to weight? Another good question, I wish I knew the answer to it

w/ age your metabolism slows down a bit so it's hard to lose weight. i remember i eat twice as much 10yrs ago than today and still have rock hard abs w/o even going to the gym.

Mostly Natural growth most of the time, What do you mean exactly? Is there a tendency to put on fat with age, which means a fighter has to move up or something else? unless you have Ariza's shakes. No comment:D

Mostly Natural growth most of the time, unless you have Ariza's shakes.