I think it's a combination of shape, size, clamp pressure, pressure distribution, and each person's anatomy. I was wearing my Sundara today and I found that the clamp pressure was actually worst on my cheekbones in front of my ears. The size of the Sundara's earpads places pressure on a part of my cheekbones that I'm sensitive to and thus causes more discomfort.

In contrast, my Shangri-La Jr is vastly more comfortable than the Sundara because the earpad is slightly larger, and hence the clamp pressure is distributed over a larger area, and the areas in contact are different, and those are areas that I am less sensitive to. Also, the SGL Jr has earpad swivel, which the Sundara lacks. This means that the earcup will swivel to more evenly distribute the pressure towards the back of the earpads rather than the front which makes contact with my cheekbones. In that sense, perhaps a headband replacement would work, as it could add the earcup swivel that's missing on the stock headband.

Unfortunately for me, I tend to find many headphones that others find comfortable to be uncomfortable on me. The Philips Fidelio X2HR was reviewed to be very comfortable, but I found it so uncomfortable from both a hotspot and clamp pressure that it made me see stars from wearing it, and I had to return it due to discomfort. The Aeon 2 Closed is also rather uncomfortable and causes weird pressure on my temple, despite not even resting on my temple. Focal headphones in general are incapable of properly distributing downwards weight across the top of my head, and immediately form a hotspot. I guess I have a narrow and pointy head, so many headbands don't have enough curvature to properly distribute weight across my scalp.