The important thing is that they are elastic - they don't stay stretched. They're not like sweaters that stay stretched out forever after someone large wears them, more like rubber bands that stretch temporarily before returning to their typical size. I think that's why OP used the term "stretch out," as too many people seem to think once a vagina stretches, it's a permanent change. It's such a weird misconception.

Giving birth stretches a vagina more than any penis (or fist) ever could. Though birth can make some changes that last, it's more like "Now I need a larger size menstrual cup" than "Now penises just slide back out," as the popular belief would have people believe.