What is the psychology behind male pornographic actors having larger than average penises?
Besides the simple mechanics of it being easier to position actors and film the details of intercourse there is probably a supernormal stimulus effect. The term, coined by Tinbergen (1948) when he observed birds laying artificial eggs of ridiculous size, is used to describe the effect of a stimulus which elicits a response more strongly than the stimulus for which it evolved. Very large breasts, buttocks, penises, tall heights, etc. are not necessarily particularly useful but because we evolved to prefer larger values of these for fitness reasons we suffer from a supernormal stimulus effect when they are exaggerated. Imagine the rarity of such things for millenia of our evolution during which food and leisure time were relatively scarce. There was little reason to evolve a preference against extreme proportions.