We've done CC on the Magic, Wonder, and Fantasy. Yes, it can technically be more crowded on the bigger ships, but it's well designed to handle the crowds, for the most part.
Some suggestions if you do go on the Fantasy or Dream, would be to do whatever you want to do most first thing.
I love to snorkel, and it's fun to hunt for all of the underwater 'treasures' in the snorkel lagoon. But I only snorkel first thing, if the water seems relatively clear. Because it's a Lagoon, the water can get murky if there are high winds, heavier surf beyond the Lagoon, or lots of people splashing in the Lagoon. For that reason, I don't book snorkel gear ahead of time, but rent them day of, depending upon the conditions. And I don't go snorkeling again after lunch.
If you like the idea of hanging out at Serenity Bay, it's likewise nicest in the early morning, when it's less crowded and easier to find lounge chairs, especially shaded chairs.
Afternoons, we generally just play around at the family beach, listen to tunes, build sand castles, ride the water slides, etc.
We love the classic ships, but I'll admit I'm looking forward to sailing on the Fantasy again.