Who cares if she would change her body for you? That isn't the issue. Part of loving someone is accepting them for what they are. And wanting them to be happy. If she can't accept you for what you are, and what is important to you, she ain't worth it. Sounds quite self-centered to me. She wants you to lose muscle to make HER feel better. Whatever. This is why girls get a bad rap.
Like jbuts said - would she gain 25 lbs?.............of FAT?