I don't like it if games like this were exceptions, then okay, but in almost every game, the same kind of dick.
So when you see any change at all, you're already surprised.
The problem with realistically sized cocks in porn games is the graphic fidelity. If you have a normal dick, in most games, it looks like a finger or a flesh coloured dildo. It is way easier to give it more noticeable girth and veins and in general texture if they are bigger.
Same goes btw. for animal dongs in those games, they are also way bigger that your normal counterpart.