The industry allows for a normal shrinkage of two percent.  Shrinkage beyond this is due to poorly stabilized materials.  To be sure, measure the collar and sleeve length.  Measure the collar from the beginning of the button hole to the center of the button.  Measure the sleeve length in a straight line from the center of the back of the collar to the end of the cuff.  If these measurements correspond to the shirt size, it has not shrunk.