wow.. i don't think friends like that should be considered friends at all. women come in all shapes and sizes...it's part of nature. and the idea that "curvaceous bodies are better" is JUST an opinion, not a fact. just like some men prefer heavier women with larger breasts, some men prefer thinner women with smaller breasts.. and any other variation, at that. WOMEN are just overly critical about other women, even more so than they are about men. it's just a crazy phenomenon.
personally, i think your friends might be a little jealous of you. maybe they wish they were as thin as you, and want to put you down for what they think you're lacking. or maybe they're comfortable with themselves, therefore believe you should be more like them. whatever the reason, i think you should embrace your body and love it because it's YOURS, girl! i'm pretty small chested myself and i used to beat myself up about it all the time because i felt less attractive than girls with larger breasts than mine.. but i've learned to love my itty bitties because nothing's gonna change em. well, they'll change over time by natural causes lol but for right now, they are what they are.. and i accept them.
you are every bit as attractive as any other woman out there with large breasts, small breasts, no breasts.. love yourself for you (and for those boobies)!!