.
She is not fat, but she is fishing for compliments...I know of a woman who is so confident of her body, she says "Gawd Damn, look at those curves!", and let me tell you, baby got some dangerous curves. A woman who talks about how fat she is to her boyfriend is only asking for reassurance that she is not, she is putting her self esteem in your hands, and how you view her...basica insecurity.
Tell her, that when she gets fat, that you'll let her know, until then she needs to not focus on on such (which is a nice way of saying STFU).