People's bodies are weird.
My Mom and I are the same height and same weight, (5"2, 11st) but she takes a size 16 and I take a 10. You can only judge it on yourself really! I know that I have a really narrow waist and massive hips, so if I buy a flared skirt in an 8, it will fit me (size 8 skirt fitted me even at 13 stone), cos it will just flare out over the hips. Pants are different though as they have to go over the enormous arse lol, so I'm a tight 12 in pants!