Dress sizes mean nothing to me. I just say skinny (looks like a stick), slim/average, curvy (slim/average with bigger tits), chubby (a little fat around the belly/chest/legs), fat (obviously large), bbw (more horizontal than vertical), and then morbidly obese.
Much of it comes down to height vs muscle and fatty weight. Dress sizes are affected by a combination of height and weight, so a girl won't be a size 6 if she is 7 feet tall and average shape.