I ordered a XL and received a L.
They must have been out as they pasted a XL label over the L shirt package. But it was visible. Groupon wanted me to send back & take pics & send in. I'm not one to mess around much with phone pics & mailing them out. So I just kept the L & started using it. It was of course pretty tight but maybe that helped, I don't know. Anyway it does make me sweat more which is why I purchased it. I jog in it & sometimes wear it to the gym.