Vaginas change through LOOONG periods of time, not because muscles stretch, the main reason is that the spongeous sides decrease in size as they get older. I'm 43 and hung (8.5 long, nice proportion girth), every girl I fucked back when we were 20s and fucked her back at 40s as a "revival sex", got wider vaginas, and not all of them were very promiscuous or fucked regulary bigcocks, no not all were mothers, and no, they don't hide their sex life to me, they know I'm not impressed by bigger cocks than mine, life is not like porn, specially about vaginas and penis sizes.
About swapping a big cock with a small one in a matter of seconds, porn and logic tells you the woman will feel empty, but I've found that is not true for many women. The big ones are not as incredible as porn makes you think (they are better but not perfect, they have their own set of "challenges"), they stimulate, so when the smaller one that doesnt hurt goes in and the vagina is already highly sensible from the bigger cock, the smaller one feels even better for her, specially in certain moves or full throttle, and sometimes they fire more orgasms than the bigger one that blocks female orgasm by putting her in a neurotic survival state because of the pain (spoiler: higher level of female sexual pleasure comes with the cost of accepting certain degrees of physical pain, that is a bitter truth for women to swallow). This is something inexperienced guys dont know (cough authors cough).
Other events that change vaginas size are birth and surgery, they might not come back 100%. Muscle stretching is more for women who clamp, so they can release and regain part of their potential. In order to stretch a vagina you would need real tension for long periods of time, like the african tribes doing shit with their necks or their lips, and also almost the same principle as penis hanging and stretching.
