What is the psychology behind male pornographic actors having larger than average penises? There is a question What does scientific research say about the relationship between penis size and attractiveness?

This  answer and this link give the statistics of penis size. Given these statistics, the average man performing in porn, would likely be in the top 2.5% of the population of penis size.


  Porn stars are statistical outliers, not the norm; men are more likely to be over 6’3” than to have a penis longer than 7.5 inches.  


It also "appears" that men may be inclined to having poor self-perception, regarding their penis size.


  Males self-perception
  Males may quite easily underestimate the size of their own penis relative to that of others, because of the foreshortening obtained from looking down, due to repeated observation of atypical penises in pornography, or because of the accumulation of fat at the base of the penis.[34] A survey by sexologists showed that many men who believed that their penis was of inadequate size had average-sized penises.[35] 


Given that men are the main consumers of pornography, why does pornography feature so many men, out of the norm for penis size?
Would it be more appealing to men, if the men starring in porn, had average sized penis?    what is the psychology behind using men with such, abnormally, large penis?
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.