When a girl tells you something bad about her phyisical appearence such as "I'll fight this cold with my big ass and my fat" or "Do you think I am fat?", etc etc

In my opinon a person that says this kind of things whats to be said "No, you are pretty! You are not at all fat"; but should we say that to women or there is a better answer?

BTW: she not fat, but she is not skinny