As someone who has dated (and married) "big" women, I can say that this wouldn't be an issue for me. Honestly, if my wife decided to change her gender, I'd probably find something attractive about her physically because I'm so attracted to her emotionally/intellectually. Then again, I'm kind of attracted to all sorts of different women for different reasons. It took my wife a little while to get used to it (how can you think that person and I are both good looking?!), but eventually she just accepted that I like "women" in general and even some men. I'm in a monogamous relationship (which is what I want), but on a fantasy/psychological experiment level, I could conceivably be with lots of different types of people.
A couple of points: 1. Not to be cynical or mean, but chances are your girlfriend isn't going to lose the weight. It's really really hard. Without drastically changing diet , you have to rely solely on repeated exercise to keep lost weight off, and that could get interrupted by injury, pregnancy, or any number of barriers life presents. Eating is a major part of the lives of overweight people to relieve stress, have fun, or just spend casual leisure time. It triggers a pattern of hunger and satisfaction that's really hard to get out of. But, I hope she does lose weight (and my wife too, some day) for the sake of her health. But, you cannot bring this up or be discouraging in any way. It's a really hard position to be in, especially if you're thin with little to no effort, like I am. This will most likely lead to crying and frustration, but that's life. Try to be as supportive and patient as you possibly can, which it sounds like you already are.
2. I feel like people on the internet put too much stock in their sexual inclinations (boo, hiss from everyone, I'm sure). My wife and I had very specific sexual fetishes before we met that we experimented with while dating. After a while, we just lost interest in them. The intimacy of physical contact was simply better. I mean, when the lights are out or you're very physically close together, can you really tell the other person looks like? The rest of time you're just hanging out and talking, which is something I don't need to be especially physically attracted to someone to be able to do.
