I thought so too. I have one that's 65mm and it barely fits. It's just enough that it cuts off circulation, which is bad! 5mm more would be perfect.
I have been trying to fit chastity devices and I did all the measurements. I need one with a 70mm diameter on the ring part in order for it to fit...
I'm new here, wanting to try chastity for years now but none of the devices fit :(