Because it will have been fat since puberty or more likely beforehand, and thus the fat will have fucked up her hormones so she didn't grow any. My fat little sister is very much the same, when every other woman in my family is a decent D cup or above, she is about a 38C, much of which is fat, and so if she lost the weight to be the 32/34 that most of us are she would probably be rather flat chested.