(X-MEN) The reason Wolverine is physically smaller in the original X-MEN films but super jacked in the other films.

TLDR: The more damage he takes, the more his body compensates by building more muscle. In the first X-Men, he has not been in heavy damage situations for a while, negating the need to build more protection.

So i know the official out of the universe explanation is that Hugh Jackman did not have the time needed to get in superhero shape. But here is a probably ridiculous in-universe explanation.

So before the events of X-Men, Logan looks to be a traveler, lying low and having problems remembering who he is. He is doing some bar brawl cage matches for extra cash, but he is taking little damage from his opponents.

Compare that to the years before in other films where he is constantly being shot ,stabbed, blown up, and what have you. The more punishment he takes, the more muscled he is, like his healing factor is more triggered by more damage,rebuilding him bigger and stronger. If you look at the films where his is fighting more often, this is when he is at his biggest

He could even shrink in size for his body to conserve energy during times when he is not subject to constant damage.

https://www.vulture.com/2014/05/hugh-jackman-reviews-for-first-x-men-movie.html