OK In my limited experience.....
I think its how tall a woman is.
I have done a woman almost as tall as me and she had a big Clout but then she was almost 50. Great shag though
Had children.
Then I did a Teenager Same Size but also getting on for 6' with a big clopper. Also a great shag.
no children
I have also done young 20ish somethings with fannys like mouses ears.
And a 60ish woman with children who was as tight as a nuns c**t She was 5' and a fart.
So my conclusion is that the size of the fanny is only related to the size/height of the woman
N