I'm 5'5" and 115lbs - the size Small fit great around my waist and hips, but were weirdly loose and too tiny/thin at the crotch - I could feel if I separated my legs too much (like you do 100% of the time when swimming) the whole pool would be getting a bit of a view, if you know what I mean.  Frog kicks would be out of the question.  Guessing it has to do with a high butt-to-thigh ratio.  Returned.