It's an older slang usage of thick. I believe they use it more in the UK than the US now a days.>>83627897
Thick means a shapely body. It's another black colloquialism that gain popularity so everyone started using it too. You are right in some aspect many people misuse "thick" when referring to women who are obviously fat/obese. Go to any board that has threads about "thick" women's figures, it will start with a woman with a curvaceous and proportioned body and you will without question have it running over with fat round women with rolls in no time.
It doesn't help that fat women like to use cute terms in place of saying they're fat. So it muddies the waters elsewhere.