I think you feel guilty because obesity is unhealthy and you indirectly encourage it because of the fetish. But it sounds like she's doing it of her own free will and that it's good for her mental health. I believe that mental health is the most important thing as long as the physical health doesn't suffer too much. It has to be in balance. So if she has an active lifestyle and doesn't only eat fast food, I wouldn't worry. so just enjoy it. I also think it's good that you put her well-being above your fetish. I know from my own experience that we sometimes find ourselves in a moral conflict because of the fetish. but as long as she‘s happy with herself, you dont need to feel guilty!