I totally agree. Max stretched vaginal depth does correlate with the inner pelvic dimensions. An old name for one of the measurements was "coital diameter", but that term is no longer used.
It does happen that some short women have a deep pelvis, and some tall women have a small pelvis, but it's less common. That's also why people think body size doesn't correlate though, because they know of exceptions.
Vaginas stretch up and slightly to the rear. The upper back side of the pelvis ends up being the actual limiting factor, even if it feels like to her that cervix discomfort is the limiting factor. The front vaginal wall does stretch a good bit too, but not as much as the rear wall.
"The posterior fornix can be displaced clear to the sacrum", wrote the gynecologist Robert Latou Dickinson in his book Human Sex Anatomy.
