That's not overweight. Actually I think that's really sad you feel that way. Women have enough pressure to be thin seeing models and celebrities and it's depressing to feel like your not beautiful because your not thin. What if she get's pregnant? Her body changes and it will be hard to get the baby weight off. Are you gonna be turned off more. A women's beauty isnt her apperance it's her thoughts, her personality, the way she treats people, and the way she carries herself. Maybe she hasnt tried because she knows your not attracted to her and maybe inside she's depressed. Talk with her not about how you feel or your need for her to be thin but ask her how she feels about herself. Suggest things that arent overwhelming. Sometimes it's hard to make yourself go to the gym. Maybe she could do a video at home theres this one by a mari winsor it's pilates very good excersise and it's not long or hard you see results fast not just in apperance but in how you feel it takes your stress away and makes you refreshed. That's just my oppinion. If she dont wanna change tho and you cant handle it dont cheat just let her go cause there is someone out there who can love her for all that she is no matter what she looks like.