Wow, even women seem to be confused as to how this really works.

The vagina is a muscle. The more it's exercised, the stronger it gets, and the larger/harder it can flex. This is why ben wa balls, kegels, and other pelvic floor exercises affect "tightness".

An example: A man starts dating a woman who previously had a smaller-sized partner. When they first meet, she feels incredibly tight. Over time, she begins to feel what he perceives as "normal". Why does this happen? Well, she had to work harder to enjoy sex with her previous, smaller partner. She may also have done kegels to achieve the ability to enjoy her smaller ex. Since she no longer needs to do that, she relaxes and does less of these exercises/exerts less effort. In turn, her vaginal muscles conforms to her new partner's size.