My 2/19 Fantasy sailing was about half full, so it was like a Magic/Wonder amount of guests on the larger ship. It was noticeable, but not like "this place is empty" -- it more depended on where I was.
Cabanas and the dining rooms still seemed busy to me, but if you looked closely, you'd realize there were fewer tables with more space between them. Normally in Cabanas, you could have trouble getting out of your chair because someone else was right behind you.
They were limiting seating in the Walt Disney theater (three seats between parties, skipping entire rows) and so that limited capacity for shows. They ran each show on two nights (Aladdin, Frozen, Believe) so you could try for either or both.
Mickey and Donald pools were limited to 12 or 13 bathers at a time, for about ten minutes each. (We thought it was actually a really good system.)
We got to ride AquaDuck multiple times this cruise since the line was short, especially like around 4pm even on a sea day. Last time on the Fantasy (way back in 2013), I think we got onto AquaDuck once because it was so busy.
Debarking for Castway, the stairs to the midship gangway was a jammed as ever despite the virtual queue system on the app. We went to the aft gangway instead. Once on the island, we're used to a Magic/Wonder amount of people on
Castaway Cay, so our day there felt normal by comparison. "Busy" but we know it could be a lot more crowded with a full Fantasy.
In all, we had a wonderful time and we felt lucky to take advantage of the Fantasy with the amount of people on board.