As Irene shows us, the longer a sissy is locked up, the smaller her clit gets. I used to wear a CB-6000 regular as the S size did not quite fit. But after time, we were able to switch to the CB-6000S. And then I didn't even fill that anymore (we currently use the smallest Holy Trainer).
Source | http://silkyirene.tumblr.com/post/133232728487