She is selling the BBW niche, get that through your heads! She is obviously comfortable with the way she looks, and although it's not healthy, there are lots and lots of people who happen to think it IS attractive. Why should she lose weight if she's happy with who she is and doesn't have any health problems that warrant it and lose her niche? If she lost weight she'd be one more amateur in a sea of amateurs, and there happens to be a little less competition in the BBW niche, anyway.
Just because you don't find bigger women attractive doesn't mean you have the right to insult those women or the folks who think they're attractive.
I don't know what you look like, but you might have acne, scarring, a flat butt, a tiny penis... anything. But I wouldn't make fun of you for it, because that's who you are and that's fine with me. What I do find hard to accept, though, is your attitude. You really should be a little nicer to people. If you really wanted to help her with your "advice" I'm certain you could have found a more delicate way to put it.