I don't think you can go by sizes either. My friend is 5'1 and about a size 6 does cross fit and she looks thick but fit. I am 10/12 and I don't think I look thick, I now (unfortunately) look like a thin pregnant person. Which means I definitely need to start doing sit-ups. Haha
Thickness has to do with where you carry your fat or how muscley you look. Or where/how your "junk" (whether it be fat or muscle) forms.