Oh my... In few years time you'll see how silly and unrelevant this situation was, promise.
4 years ago I had a massive crush on a guy I went to school with. We were getting along very well, he knew I liked him... Little did I know that he was in love with someone else and now, 4 years later they're engaged. I don't like her, I never liked her and personally never seen what he likes about her (we all went to school together). But it's not me who's in love with her so it's not my business...
Now, 4 years later I have amazing boyfriend with whom I live and talk about the future. The memory of that "crush" is just that - a memory.
You sound a bit mean, to me at least... Obesity has so much more in store than just eating. I put on quite a lot of weight since I've started uni - would be a fool if I said that mental health and health issues didn't contribute to it. I'm working hard to lose that weight and restore "balance" in my life.
What I want to say is, unless you know that girl personally, you have no idea what made her that way. I know a girl who became obese after she had a miscarriage... Another one has scars from cuts all over her hands... Everyone has some struggles. She might be a wonderful person, for all you know... Remember what they say, looks fade but personality stay. Like people have said here - it was clearly not meant to be. Doesn't make you a terrible person. Just not the right one for him. Or him for you.
One day you'll meet someone who's going to fall head over heels for you. With the feeling being mutual. I like to think that perhaps "destiny" is a thing - I believe there is something to it.