well HW was 5 leveling dungeons, 1 MSQ dungeon, and 2 tome dungeons. (3 trials)
so far we're short 1 leveling dungeon, as there will only be 4 leveling dungeons, and we know we get 2 tome dungeons. we dont know if we get a MSQ final dungeon. (and we only know of 2 trials.)
We get 1 main city and the new revenants toll.
we get 6 zones, just like HW.

So for the file size to be so different compared to HW, maybe the amount of voice acting is lowered, and there's a lot of reused assets maybe.
(such as oriental styled building being reused often enough to save space.)
Unless just 1 dungeon makes that much of a difference.