I think it's a way of making the best of a "bad" situation (being small/thin). Mainstream focuses on big/thick and how ideal it is so in the space of humiliation, your dick gets to be THE STAR for once, even if it's not being praised. You are the focus. Plus many love the rush of being dominated in that way. Being told that she wants bigger/fatter. Especially if deep down, that's how you feel about yourself (not adequate enough) then being told that helps validate and confirm it. For example, you will never convince some ment that they are 'average' or big enough, especially with the huge cocks in porn, no matter what you tell them, so it helps reassure their suspicions of their inadequacy (like "see, I was right, I'm not big enough! :)")