Not all of their products are created equal
I have previously purchased XW-X Insulator Expert tights and they were phenomenal! I decided to buy the Performx and check it out. I am 5'1" and XS on the Insulator fit perfectly, so I ordered the same size for the Performx. Unfortunately, when they arrived, I could barely pull them past my thighs. They were a lot smaller!
I'd highly recommend you try them on at a sports store before purchasing online!