Society, media, pop culture, you name it. Though for some of us it goes a lot deeper, too. Some of us end up with parents or grandparents who tell us at age 3 "No one likes a fat baby" - some of us are teased merceliously for years and never completely get over it.
As for me, I've battled with anorexia since I was 11. Nearly 12 years later I can count my ribs and my pelvic bone does stick out a bit - however there are times I'm still called fat despite being a siz 6/8 (depends on who makes the cothes). And do I still feel overweight and or generally unattractive - fuck yes. Also thanks to society, the media and jackasses like those your gf's run into on Yahoo.
Anyway, like everyone else has said, don't stop with the compliments. Don't entirely shower her with them either though because then she may think you're just saying those things just to make her feel better and given she may feel that way already, you don't want to add to it.
In the long run all you can really do is love her for her and let her know that - it's up to her to believe it and accept it