I am right at 39”.
I thought my medium belt was a little small. The padding doesn’t seem like it wraps far enough around my waist and I have to let all the slack out of the belt to attach or detach it.
However I’m not sure those annoyances have really affected the overall functionality and comfort of the pack system. The frame w/belt carries the weight really well for me and for the most part stays put under load.
The benefits of the large belt with more padding may translate to added comfort and the additional mollie webbing column can’t hurt.
I ordered the large belt but haven’t had a chance to put it on the frame yet. By the time hunting season comes around I hope to have slimmed down and will probably be ok with the medium. When I get the large belt installed I’ll report back.