sudas: It sounds like many of the women who contribute here are monogomous. It would be interesting to hear from women who regularly have two guys: one thick, one thin. Can the thin guy tell how often she's gone with Mr. Big? If Mr. Big hasn't had her for 6 months, does she feel tighter to both of them?
I did hear a story of an almost-teen girl who found a "gag" gift; a dildo in the attic from her mother's wedding shower. She learned to take it, but later discovered that it was much bigger than that of a teen boy. When she did have sex, she could hardly feel him in her.
I understand that Kegel exercises can tighten or relax the vaginal muscles. There's even pussy weights one can use too. However, there are deep parts of the vagina, beyond the muscles, that might stay loose. Advice to premature ejaculators is to put it in deep and don't move, so her muscles don't get you off too soon.
I suspect that if a woman regularly has a big one (penis or dildo) it will stretch her. If she switches to a thinner one she should, over time, tighten up to fit.