I sent an email to Sennheiser's tech support and asked them specifically about a larger replacement ear cup for the Momentum.
And here's what I got in reply:
Thank you for contacting technical support, at this time we do not offer a a bigger option for this model, Germany is working on this an it will be probably happen early next year. please keep checking to see when they become available.
Carlos Zambrano Sennheiser Electronic Corporation Sennheiser Technical Support Specialist
I hope that the "bigger option" is a replacement ear cup part of some sort and not an entirely different model...I JUST bought my Momentums yesterday and love them but would prefer to have a slightly larger ear cup for my male adult sized elephant ears.