I followed the advice of the other reviewers and I bought an S, even though I often need an M for my capri workout pants. These pants are the softest ever, and this is really why I kept them and chose them over others I tried. I wish the waistband would be even a little wider, so to give that "tummy control" effect, but it is wide enough not to cut into one's belly, so no bulge and no falling off.
I am 5,7 and 140 pounds, with a tad of post-baby belly, and these worked very good for me.