When Chyna first debuted in WWE in 1997, she was a first in wrestling - a female bodyguard who was more physically intimidating than the men she seconded. While Triple H (and later, Shawn Michaels) didn't cut the most imposing profiles, Chyna's huge muscles ensured that she was taken seriously by all of her enemies.
As Chyna became a more popular babyface, though, there was pressure on her to adhere more to the "WWE Diva" standard of beauty. She got extensive plastic surgery - including breast implants and a shaved jaw - to make her look more traditionally "feminine," and she also slimmed down quite a bit, losing a lot of muscle mass.
It's possible that some of the weight loss came about for reasons of athleticism - it was around that time that Chyna started to regularly compete in matches, and the demands of pro wrestling and bodybuilding are quite different. Still, the fact that she posed for Playboy in 2000 - an extremely popular edition of the magazine - shows that much of the change was about appearances.