Well, EVERY cruise ship, across all cruise lines, is smaller than the Oasis class ships. π The Odyssey is the largest Royal class right behind the Oasis class. They are still huge. My personal opinions is... if teens can't find enough to do on either ship then cruising isn't for them.
Both classes have plusses and minuses and I'd have a hard time picking a winner. I actually might like the AMPED Freedom and Voyager classes better, and our teenage kids love them all.