I am 5 foot tall about 110lbs give or take.  I wouldn't consider myself skinny in the middle.  That said, I usually veer towards 'Mediums' for fear of something being too tight.  I got the small figuring that I am technically a small.. well, the small has some space left.  It is quite airy.. but the string ties are great because you get control.
There's lot of space in the legs though, obviously these aren't 'fitted'
I got them for the gym and I will use them fine, but I probably won't be wearing them on 'leg' days as I would fear I'd flash people..
BUT I haven't run them through the wash yet so maybe they'll shrink a little?