I would say you're neither a slut nor a chubby girl.

You're a beautiful chick who's trying to do right for yourself and for those around you.
It may be hard, but I think the best thing to do would be to stay friends with this guy and don't let his relationship consume you like this. Be confident with who you are! It sounds like he likes you fairly well for who you are. It's probably impossible to say if he views your friendship as platonic or potentially more than that, but since you know he's in a relationship, try your best to let things be platonic. (You don't want to make girlfriend angry, and you don't want to come off as too strong or desperate.)
If he really does like you as more than a friend, it's up to him to decide what to do. If you get *clear* signals from him that he's interested in something romantic, then it's time to talk about it with him. If your friendship continues without becoming romantic, then you should feel happy to have such a nice guy as a good friend, and congratulate yourself for handling the situation appropriately. And also understand that if he isn't interested in romance, it would never work out between you anyway.
But more than anything, be proud of who you are, and know that even if you don't have this guy as a love interest, it won't ruin your life. There are so many other good things in life besides this one guy.

You are smart, beautiful, kind, and you deserve to be happy no matter what!