I'm 5'4" and 170 lbs and I easily wear a size 10 dress. Actually, I have a dress that's an 8 that I can still wear too. It all depends on the shape of the person, the clothing and the brands you are buying.
Yes, it completely depends. I'm 5'6" and for most of my adult life until a couple of years ago I weighed around 145. I was between a 10 and 12.