Why porn stars have abnormal penis size or vice versa?

It's for show. This is for entertainment and for show. The same reason you don't see short people in the nba & thin people in the nfl, & 'ugly' people in hollywood. Those who have larger than average size are going to command more attention than those average size people. Having an longer penis, also allows different positions for sexual activity, and better views for the camera.
Marketing. Consider the needs of the producers of pornography. A larger set of equipment photographs better and suggests improved sexuality. This is a select subset of the population and you should not feel it bears any reference to you or the general public.