Outside diameter has nothing to do with interior diameter, that’s not how that works. The whole “childbearing hips” idea is a myth, slender women in general have an easier time with pregnancy and birth. But also tight isn’t always bone structure at all. Vaginismus is overly tight muscles, or the skin can be tight (that’s what I had and it sucked) or being not turned on will make anyone seem tight because not open or slippery.