Actually have you guys even considered whether the girl is comfortable and liking it or not? Girls pussy can be tight either because of
1) They are not enjoying it
2) They can control their muscles or are used to doing lots of exercises
3) Age-related
4) Anatomy related (some are small, some are large

Girls being loose may also meant they are comfortable and like doing it with you, thus their muscles relaxes.
Remember, if they don't like it, muscles contracts and vice versa