I found the same as well. If I stayed on my back I was okay, but tossing and turning created drafts. I also tend to go semi fetal when on my side which place my backside hanging of a normal pad. For me, going to a wide pad or wide quilt solved it. I decided for the small weight penalty I’d do both wide quilt and wide pad. BTW, the Flex in regular width works fine. I cut an xtherm large down to 60” and now I can have wide and lighter – 15.9oz.
I also picked a a WM Astralite – 16.5oz in the reg length and even wider in the hips and shoulders but pretty close (tight or just thermally efficient?) from the knees down.
For reference I am 5’9” 170lbs with a 52” shoulder girth an 32” waist with size 12 feet.