It isn't only the strength of the PC muscles that determines the "squeeze", vaginal elasticity can vary over the course of one day...but that's kinda irrelevant. Yes, some women are tighter, and some have stronger pubococcygeus muscles than others do,
but you can increase their strength, and tighten your grip, just as you can with any muscle group. Look up "kegel exercises" on the net, and you might like this article:
http://www.redbookmag.com/love/how-to-do-kegels
You may be pleased to know that vaginal pliability
decreases over time for
some women, so you may actually become tighter (childbirth may/may not be a complicating factor; many women never fully regain their pre-labor elasticity).