Final answer:
The comedian in the old comedy routine was playing around with the perceptual constancy of size by making it appear as if he is crushing the heads of people who are far away.
Step-by-step explanation:
The comedian in the old comedy routine was playing around with the perceptual constancy of size. Perceptual constancy refers to the ability of our brain to perceive objects as maintaining their size, shape, and color even when they appear differently due to changes in distance, angle, or lighting. In this case, the comedian's action of lining up the heads and then crushing them between his fingers is a play on our perceptual constancy of size, as he makes it appear as if he is physically crushing the heads of people who are far away.