I just bought-and-already-love these!
I own two pairs of the capris, and now one of these 9" shorts. My measurements are most in line with the upper range of the extra smalls (waist: 25", hips/lower-booty: 35"), but my experience with the capris made me size up to the small when I purchased the shorts. However, for the shorts, I find that I probably could have ordered my true-to-size XS. They fit well, but they have a little breathing room in the crotch, and a bit of extra room at the thigh, if I pull them all the way down. I don't think I'm going to exchange them, because I already don't want to part with them before my next run! But, I'll grab the next pair according to the size chart. :)