This, particularly the character bulking up, sounds the Guardians of the Flame series by Joel Rosenberg. The first book is The Sleeping Dragon (1983).
According to Goodreads:
It began as just another evening of fantasy gaming, with James, Karl, Andrea, and the rest ready to assume their various roles as wizard, cleric, warrior, or thief. But sorcerous gamemaster Professor Deighton had something else planned for this unsuspecting group of college students. And the "game" soon became a matter of life and death as the seven adventurers found themselves transported to an alternate world and into the bodies of the actual characters they had been pretending to be.
Cast into a land where magic worked all too well, dragons were a fire-breathing menace, and only those quick enough with a sword or their wits survived, the young gamers faced a terrible task. For the only way they would ever see Earth again was if they could find the legendary Gate Between Worlds - a place guarded by the most terrifying and deadly enemy of all... THE SLEEPING DRAGON
According to Wikipedia, of one of the characters, who really preferred the fantasy world:
In our world, he was crippled with muscular dystrophy, while there he is a powerful dwarf berserker.
This has come up numerous times before:
Book about RPG players who get pulled into their game
Name of book series where RPG players become their player characters
Characters taken back in time and into other bodies. Made revolvers to help. "Heroes" in title?
Story about group of students trapped in fantasy world by their GM
(I have not read any of these books, but I did have an acquaintance in college who liked them. I would not normally be very likely to remember much about some books I had merely heard somebody else describe once or twice. However, I remembered in this case, because of the coincidence that the guy I knew in college was also named Joel Rosenberg.)