I am not sure. Maybe that they are not constricting, and I always feel put together when I wear one. By looking at the 2 I have, at first I thought there was no way either would fit or not be tight in my regular size. I am petite and have a small-ish waist, but I have the usual cycling thighs and a bit of padding around my hips, despite my small size. I don't ever wear regular "jogging" shorts, as they make me look awful: wide. I have some longer Nike running shorts that look like bike shorts and those are OK, but the running skirt is really best for when it's warm or hot out. I'm not running now, but I wear these for walking or hiking, too.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport