If it's the costume she's wearing in the first picture yeah, that's WAAAAAY too much boob. And in some ways, I get it, because I have big boobs and it sucks that I have to wear a cami under practically everything, or that what looks cute and daring on someone else makes me look like a total whore. I can sympathize, it sucks. It sucks being insecure about your body, too.
But it's the classic case of, "don't ask for an opinion, and then bitch because it's not what you want to hear". She asked -- he told her the truth. It has nothing to do about being "big and brown". It's simply not flattering. The vampire outfit looked so much nicer, but the Storm one? So not. Your boyfriend didn't say it because he's sexist, or because he's white. He's saying it because it didn't look good on you. Suck it up, sweet cheeks. (Is there such a thing now as "small boob priveledge"?)