General

the biggest issue for her is health...as long as she is not suffering medical or real psychological problems from her gain then taking enjoyment in someone else's upset doesn't make you a bad person. whatever excites you is fine and it's not really a problem to be excited or find enjoyment in anything. What I think is paramount, however, is open lines of communication. You probably should communicate your thoughts openly and without reservation. The other issue is her health, both mental and physical. as long as she isn't suffering any serious consequences then I dont see a problem.

I think that this is the key thing here. You're not going to make her happier by coercing her to a larger size, and since it's her body, it should be her happiness that's important here, not yours.

I would not say that you're a bad person because of it, but by going against her wishes for your own desires, you are doing something wrong.

I can't say that I blame you for considering it, though, and to just think about it isn't the crime. It's going through with it against her wishes that crosses the line quite soundly.