The Momentums are semi-on-ear, unless you have exceptionally small ears I don't think you can comfortably fit them inside completely. I tug the upper parts of my ears inside and let the Momentums sit on top of the rest, this way the fit is fine for me and I can easily wear them for a few hours without discomfort.
Average ear height is 6.3cm.
Mine are 6.5cm which makes my ears slightly above average. My ears fit inside and I have never experienced discomfort.
So you don't need exceptionally small ears... just average.