Originally Posted by IBOHUNT

There are generally two different "cuts" of jersey. The "Club cut" which is looser than a "Race cut". Make sure you are looking at the right cut.
Even the Club cuts seems to run small relative to a 'real' shirt size that you may be accustomed to. Perhaps 1 or even 2 sizes.

This from the aerotech website:
"Cycling jerseys are designed to have a snug, tailored fit (this keeps it aerodynamic instead of flapping in the wind). If in doubt, the larger size will likely fit better. "