The small is a really great fit on me, but I'm a pretty slim 5'7'' guy. Doesn't leave a ton of room for bigger legs (or butts) so if you workout a ton or are generally more stocky, definitely go with a size up. But for me, they are fantastic. I never bought any sort of thermal underwear because I honestly thought they were super lame and only for dads and old people. But I'm now 31 and I freeze my butt of walking to and from my grad school campus (we're in MA, so... brrr). Guess I'm getting older but I can't wait to use these more often to keep my legs nice and toasty warm this winter.