Not a female, but I'd just like to comment on the Cigarbabe's equating size with tightness. In reality, tightness is a product of both size and elasticity. If would be quite possible for a vagina to remain the same size while losing elasticity over time. The change in elasticity would affect both the perceived tightness and the rate at which the vagina adjusted back to "normal" after being stretched during sex.
Also, there are a lot of materials that lose very little elasticity when stretched moderately, but that have critical points beyond which stretching dramatically reduces elasticity. (Some rubber bands will do this, depending on the quality of rubber used.) If you're talking about a vagina, it seems at least plausible that one would notice very little permanent elasticity change under small to average girths (and potentially substantially more), but that a truly girthy penis could begin to pass a critical point. In particular, there would come a point where vigorous stretching during sex does minor (largely imperceivable) damage to the musculature surrounding the vagina, leading to sub-surface micro-scarring that could over time reduce tightness. Obviously, the degree to which this occurs would depend significantly on the individual involved, but it definitely doesn't seem out of the question.
And, yes, the vagina is "designed" (for lack of a better word) to be elastic enough to pass a child, but in actuality, a reasonable portion of mothers experience tearing in childbirth. Once that has occurred, surgery can correct the damage and restore the original look and size of the vagina, but surgery cannot restore the vagina's original elasticity because the scar-tissue that forms around the tear and any surgical incisions will not be particularly elastic.