I am right at 155 and though I have not tried on new pants in a super long time the 12s I am currently wearing are only staying up with the help of a belt. I think alot of it has to do with body shape, I have absolutely no butt.
Check out this website, someone posted it on the forum awhile ago, it shows different women at the same weight and the various sizes they wear-
http://www.mybodygallery.com/index.html
I found this on a website, it shows that from 0-14 the difference in pant sizes is only an inch, over 14 it increases to two
Size 10 (medium): Waist = 29 inches and hips = 39.5 inches
Size 12 (large): Waist = 30.5 inches and hips = 41 inches
Size 14 (large): Waist = 31 to 32 inches and hips = 42.5 inches
Size 16 (extra large): Waist = 33.5 inches and hips = 44 inches
Size 18 (extra large): Waist = 35.5 inches and hips = 46 inches
Read more: How to Size Pants | eHow.com
http://www.ehow.com/how_5117859_size...#ixzz1CGDXk1tK