I just thought I'd point out that the number is the rib cage and the cup size is the size of the breast. Just for clarification purposes, I am a 32c, but it sounds kind of skinny, so I say 34c. A 38 inch rib cage might belong to a thicker girl. There are lots of sexy, thick girls though.
I just wanted to tell you how measurement work so you don't ask for a 38d, expecting enormous breasts on a fit girl instead you get normal looking breasts on a thick girl.