Dood the tighter clothes GIVE YOU MORE AERO~~~~!!! Ok seriously tho, the size SMALL and MEDIUM vary from company to company. Some companies I wear a size S while others I wear a size M. What you need to know is the ARMPIT-to-ARMPIT measurement for the chest mainly and if need be request measurements on the COLLAR-to-WAIST and SHOULDER-to-WRIST lengths. If you ordered from Nashbar they have a nice person maning thier online LIVE CHAT which I highly recommend you to use next time. I asked them a lot of questions and they went out of the way to verify the sizes and dimensions for me. I sometimes wonder if Preformance and Nashbar use the same LIVE CHAT person.

Just remember to always check the measurements and measure your chest all around. If you can't then take a jersey that fits you well then measure it flat on the ground armpit to armpit then multiply that by two to get the total chest size. If you take pics of the items you bought I might end up buying it off you.