When I measured my M100s, the earcup depth was only 14-15mm deep. My ears also only protrude out 15mm from my head.
I immediately noticed my ears touching the inner earcup as soon as I put them on. They became slightly irritating after 30 minutes, and very uncomfortable after 2-3 hours. I noticed if I wore them too much longer beyond that, the outer tips of my ear will turn red and I would also experience some pain.
I kept these headphones from the full 60 days, but the comfort did not improve very significantly after the first week. The clamping force did adjust after prolonged use, but the ears against the inner ear cup issue remained for me. I also noticed that there was a slight bit of variation of comfort levels between different headphones as my white M100 were slightly more comfortable than my black M100.
After doing a paper towel mod, the comfort did increased a decent amount, but I still could not wear these for a long period of time, so I ultimately decided against keeping them despite how much I loved them.