what makes medium a difficult size is the length from front to back. In the early 90's, I used attends mediums, and they fit me well because they were 33" long. The attends with waistband still is 33" long, but of course, they suck now. Point I am making is most mediums today are 30" and of course thicker. and that tends to pull the waist position down even more. So while the length side to side is ok, they are not long enough if you frame is like mine. That puts me in a large size. and some work out well some don't depending on how thick they are. thicker is better. Moral of the story, you have to test all brands to find the one that fits best.
