My opinion is that there is much more than just cock size that causes a pussy to stretch or loosen over time.
Childbirth, heredity, aging, and frequency are likely the biggest factors.
Women that take big, thick cocks often will naturally take longer to 'recover' and wear out faster than those that don't, but the process is so slow, it's not really noticeable over the short term.
I think the same would be true if a woman generally takes smaller cocks but fucks significantly more often. Basically, a gal taking bigger cocks just once a month, and a gal taking smaller cocks every day of that month (as an example) would have roughly the same 'wear' rate.
I've been with my wife for 30 years, and her pussy is much looser now rather than when we first met.
Guessing around 10 years ago, I began to notice after she came, her pussy would gape open. I could feel her grip loosening around my dick to the point I could waggle it around inside her. So I increased my frequency of eating her out so I could look, watch her pussy open up and gape, and try to close when her pussy throbbed. It used to take maybe 5 mins for her pussy to return to 'normal', now it takes the better part of a day. Sometimes longer if I've really worked her over.
However, I'd say her pussy is about worn out. She's nearing 70 and has fucked MANY men over her lifetime, in addition to taking toys and fingers regularly. Her pussy is still tight at first, but doesn't have much penetration resistance, and opens up quickly. I still have to work for 20 minutes or more to get her opened up for 3 fingers or our BIG toy , but the key is making her cum. The faster she does, the faster I can attempt or achieve 4 fingers and, eventually, my fist.