Do not forget that this is head-fi and is famous for over exaggerating the differences, both good and bad, between things. If you had a particular persuasion then you'll take what you will from the feedback, but don't go too far with it without trying. I have tried both products and have both an oversized head AND big ears. The M100s are fine; many, many hours of comfortable, non-stop listening so far!
The Momentums are basically a large pair of supra-aurals and the M100s are a smallish pair of circum-aurals. All you need to do, just as I did, is stretch the headband on the M100s a little, and boom, perfect fit with perfect clamping force. It doesn't damage them and can easily be corrected. Viva los metal-headbands!
The earpads, I had a worse time with my Sennheiser HD 25-i-IIs. These ones aren't bad and just as with any headphone, after a little adjusting, do the job nicely.
I can see you've already committed to your decision, but I just wanted to try and round out your take of "shallow cups + tight clamp" from a first hand type situation.