It has nothing to do with the width of her vagina, but rather the length of it and cervical sensitivity.
My exhusband was average (6") and he would hurt me if he was all the way in (and I had 4 children with him). I had a few lovers after him that were 9" + and yes, they would have to be very careful about the angle. The only time I hadn't experience pain (meaning their cocks hitting my cervix) is when I was pretty high and happy. LOL
PS - after I had the kids I had a hernia repaired so the doc tightened up my kitty like I never had even one baby. Ah the advances of modern medicine!