I don't think the guy did anything wrong.
Most men DO prefer slimmer women, athletic, or curvy, (like J-Lo and Shakira.) Not fat - curvy.
As has been said, the vast majority of people look better slimmer. Not necessarily thin or skinny, but not fat. It's a bitter pill to swallow, but it's true.
I myself look (and feel) heaps better at a lower weight. Like some other posters, I make more effort with my hair, make-up, and nails, and the way I dress when I am a size 12. At a size 16 (which I am currently,) and 13 stone (I'm 5 ft 5,) I look and feel dreadful.
I think 'what's the point in making an effort with my hair and nails and clothes because I am a fat pig who looks disgusting anyway.'
I am not saying everyone else who is 3-4 stone overweight is a fat pig and looks disgusting; it's just how I feel. (About me.)
As a few posters have said, don't fish for compliments by asking how you look, and then cry off when you don't get the answer you want.
And so most men prefer slimmer women who have nice nails, and nice make up and who dress nice. So what? Surely that is not brand new information? And don't kid yourself.... most women prefer men who are not fat too, who have a full head of hair, and who are well turned out, and who are solvent/rich, and preferably have a decent sized dick.
If I was on the hunt for a new man now, I would not be looking for a man who is 6 stone overweight, bald, working 16 hours a week on minimum pay in a dead-end job, and living with his parents. I just wouldn't. And I would not expect any man to be attracted to me looking like I do now. (If I was single.)
We all have our preferences. And I know that when I am 13 to 14 stone, (and size 16 to 18,) I don't ever get chatted up, or turn any man's head. When I am a size 12, and 10 stone, I get a lot more attention from men. So yeah, most men do prefer women slimmer.
I remember a woman being interviewed on tv once who ran an agency where people could meet a partner. She had 30,000 men on her books at the time, and 20,000 women. She said approximately 85% of the men put a preference for a slim woman on their profile, even those who had previously had fat wives.
Not sure WHY men are not generally attracted to fat women. I would be interested to hear why, if any men are on here. Be honest. I won't bite 
I can't speak for the others on here though!
I do intend to try and work on my weight after Christmas by the way, and get back down to 10 stone-ish by summer. DH loves me as I am, and never says anything, but I think he prefers me slimmer, like I prefer him slimmer. We both just look better and feel better. As I said though, we still love each other... But if either one of us was on the hunt for someone else (if we split, or one of us died,) we would not be looking for someone fat.