Dude. You have to separate your fetish with your girlfriend at some point.

Let me give you a personal example: I was dating a girl who was significantly heavier than she should've been from a medical perspective. I saw her, on average, once a week.

I noticed one day that her clothes were a little looser and made a comment- she loved it, of course, because who (outside of SUPER SERIOUS GAINERS) doesn't love a compliment. This, in turn, caused her to start trying to squeeze into shit that she used to wear when she was 80+ pounds lighter. It was very, VERY sexy.

Also, if you legit care about this girl, her health should come first. Sure, there are gainers who SWEAR they have clean bills of health and that they're the epitome of healthy people while also being 300+ pounds, and I'm sure that's possible, but like... I would rather be with my partner for the next 50+ years than have her stay at her highest weight and not be able to walk in ten years.

Balance your fetish with your actual relationship. Don't be an awful person and just use her as an object. She's a person too.