I'm going to tell my girlfriend her boobs weren't big enough to cosplay a character
That seems like a really bad idea! Do you really want to say her body proportions are too wrong to cosplay a character?
Yes, I want to be honest with her.
It's a bad idea to ever imply to your girlfriend that she isn't attractive enough, or that her body isn't ideal. If you want her costume to be authentic and believable, then you should emphasize that the fictional character has unnaturally large breasts, and that she would need to pad her figure to mimic it. That way you can be honest, and not make your girlfriend feel inferior.
WTF? Why shouln't I be able to say it?
Telling a partner that their body proportions aren't "enough" can make your partner feel self conscious about their body. Let them cosplay whoever they want without needing to critique their body. As long as they're having fun it shouldn't matter if their body is "perfect" to play a character.