well, you just described my dd. I can't use fb on her because they fall down on her waist so they sag between her legs. Smalls are too small, but mediums I can't get tight enough. Her thighs are chunky and the thigh snaps of an fb are either too tight, or too loose. I use HH on her and the fit is perfect. She is about 19 pounds and is on the lower end of the mediums. At the rate she's going, she'll never get to a large :LOL
hth