Chastity Sizing Questions

Hello, I'm new here and decided to make an account since I can't find a reference to some questions I have anywhere on the net. I've been playing with chastity off/on for 4 years, both enforced and not. I currently have an MM Jailbird with two rings: 5/8 OVAL GAP 3/8 (Small) and 3/4 OVAL GAP 3/8 (Large). They are both right next to each other in terms of size for the diameter.

When I wear the small one, I get this sharp pain in my lower back when I get erections and it makes me feel like I'm about to have a bowel movement, but I'm not. It does fit nice and snug though and doesn't move much throughout the course of the day. When I wear the large one, I get no real pain, just compression; however, it moves more and I find myself having to adjust it more frequently. Does anybody have an idea of why the pain occurs for the smaller ring?

My hope would be to fit into the smaller ring without the back pain as I feel like that is the appropriate fit. I don't want to adjust on the gap though as I feel like that is the ideal size there. I'm thinking about ordering another device in the future and I would like to get the right fit on the first try. The only other thing I'm thinking that could help would be to get a smaller cage. For this device, I copied the specs for the cage on the CB-6000 since I didn't have an issue with that one.

On another topic, I also feel like veins are a lot more prominent in that area ever since I tried chastity and they won't reduce down even if I take a significant break from it. I'm wondering if anyone else has experienced that as well.

Thanks in advance for whoever decides to reply.