I don't have any experience with the CSM, but I can tell you about belts and skinny people...
I'm 6 feet tall, and at the moment, I'm down to about 130. I'm a rail.
I wear my pants such that my belt sits just below the top of my pelvic bone on the side of my body.
In that position, some belts can be uncomfortable. I'm essentially completely flat across the front of my body, and when you get to my side, it makes a pretty drastic curve around the corner to my side.
Very stiff belts can be uncomfortable. For a long time, I've worn a duty belt, and it tends to push in on the very front of that pelvic bone, which gets uncomfortable. Bending exacerbates the issue.
I just got a Rock Steady gunbelt, by High Noon Holsters. Very stiff belt, 1 1/2 inch wide leather. So far, no complaints. I can feel it hitting that same spot, but some belts bother me worse than others. Dress belts with the little slidey-lock buckle are some of the worst I've ever worn.
I've also had a reinforced tuckable IWB holster in my belt at 130 for the last two days, making CCW Dry runs. No big complaints with that in my pants either.
So, yeah, if you're a beanpole like me, many belts aren't very comfortable. It's tough to call ahead of time. I have to wear them a while to really find out. So far, I've had the Rock Steady on for a week, and I like it a lot so far. No discomfort at all.
Hope that helped a little bit...