Brant, first of all, I love your name, off topic I know, lol.
But you bring up some good points actually. Medically speaking, I am not quite sure how to say what I want to here, but, a woman's pelvic muscles that surround the cervix sort of remind me of a sling shot. Meaning that the muscles will stretch allowing a man who is longer to fit.
I read once, or Doc told me that the average length of a womans passage is between 5 and 6 inches, but due to the muscle structures surrounding, will stretch, which allows a longer man to get all the way in.
As I said before, Doc is bigger than average both in length and width. He is able to hit my cervix, which excites the heck out of me. And truth be told, of the few times I have been able to climax through vaginal sex, it's been because he's been hitting on or close to my cervix. However, I have also climaxed vaginally when the penis was hitting a different spot, but it wasn't nearly as intense, and felt totally different. I think the reason doggy style is by far my favorite position is for exactly this reason, along with feeling dominated, but it allows him to get even deeper, which in turn, makes him thrust even harder into my cervix.
Mmmmmm!