I'm not sure, but some of them do seem to run a size small. I measure 32 inches around just under the armpits, but my Perfecto (pictured) is a men's 34 with the side-laces and it fits just right (and it's a good thing too--it was the smallest I could find!)--I can fit a couple of sweaters underneath, but if I don't it's still snug enough that it won't let the wind in.