The partial smocking at the back provides a lot of stretch. Initially, I ordered a 2X because I measured at my belly button (turns out you're supposed to measure where your waist creases when you turn to the side). That ended up being way too loose, so I sized down to a XL and it fit perfectly!
Size chart states XL at 33", but my waist is 40" when I measured properly, so this skirt provides a lot of stretch. Now I have something fun to wear to work!