The rumor is probably based on the fact that when you work large muscle groups, they demand more of your blood. It rushes away from other areas. Post and mid-workout, that gives some guys a small flaccid.

That is part of why some bodybuilders looks small in their modeling shorts at bodybuilding shows- they "pump" before they go onstage to make their muscles nice and full of blood. Pumping isn't done with a pump, rather, it refers to pumping blood to the muscles, through weightlifting.