I've wondered about this, too. At my heaviest, 246, I wore a size 24 jeans. At 230, when I began Phen, I was in a size 20 jeans. I carry my weight around my middle, thanks to PCOS. If I go by a sizing chart, my hip and chest sizes indicate I should wear a 16, but my waist size indicates I should wear a 26!
My mom used to make clothes for me, but they always ended up too big if she picked the pattern size based on my measurements. She finally got so frustrated that she gave up. LOL Now, I just try everything on. The brand, the cut, and the fabric determines the size. At this point, that ranges from 16 to 18. I'm 5'7", btw.