Consider passing (childbirth) something the size of a small Watermelon through an opening with a diameter the size of toilet roll (the cardboard roll part).
No matter how elastic the roll (vagina) may be, it's not going to be the same. It may reach back "close" to the original diameter, with Keigal exercises, but not the same original diameter.
Hence, different fitting sizes for THE KEEPER and other devices for those who have given birth and those who have not.