Two reasons, mainly. First, I wanted to keep it at least somewhat realistic as I firmly believe that the best works of fiction are, at the core, more or less believable.
The second reason is more of a practical thing; I wanted to have all those lovingly nasty details in my scenes - clear-view penetration, gape, proper creampies (and cumplay), and everything in between. The same reason why I go to such lengths as to simulate/model a lube in Iris' scene or saliva in Ginny's. Some of these things get increasingly difficult to do with more unrealistic shapes and sizes, especially on the penetration front.