In regard to the vagina, it is important to know that vaginas can change in size during sexual excitement thanks to a pretty amazing process called vaginal tenting. What happens is this: vaginas start out at about 3-4 inches in length (during an unaroused state).
During sexual excitement, muscular tension pulls the uterus upward, which has the effect of making the vagina both longer and wider. Now, instead of being 3-4 inches in length, a vagina may reach approximately 5-6 inches in length big enough to more comfortably accommodate penetration with a mans penis, a sex toy or a partners fingers.
When a womans body returns to its resting (unaroused) state, so does her vaginal size. The vagina does not stay tented. Even so, sometimes a womans vagina may feel tighter or looser during vaginal penetration, just as a mans penis might feel bigger or smaller to his partner. Often this change in perception is due to a difference in lubrication.