Bethesda's got some kind of top secret compression technique. All DLC on Xbox must at or below 2GB. It'll be larger than Dawnguard, and I got waaay more hours out of Dawnguard than I did Pirate's Booty. I think I got more hours out of Hearthfire than Pirate's Booty. They're two different styles of games, and neither of them make for good comparisons. Just compare it to Dawnguard.
Dawnguard is supposesdly half the size of Dragonborn.
So, maybe infer getting around double to 1.8x the amount of hours you got from Dawnguard.
Dawnguard didn't add much land mass though, which is why I'm really confused. You had the Soul Cairn as a new area, but it was pretty much empty and you only were there for like 3 hours tops, and let's all be honest, no one goes back to the Soul Cairn once you are done. The only other big area was the place with Auriel's temple (can't recall the location's name) and then the two keeps. I don't know what the total landmass for all of that added up would be equal too, but it didn't feel that large. If DB is adding a whole new island with lot's of new stuff, one would think it would take up some space. I mean, didn't Shivering Isles take up a few gigabytes?
If it has something to do with how Bethesda is compressing the files, then that would make sense, but just comparing it to Dawnguard... I don't know. I had fun with Dawnguard, but I tend to RP and take my time. Even doing that, it only lasted me 20 hours tops, and a lot of the Dawnguard content was underwhelming to be entirely honest. Solid storyline yeah, but afterward I felt like I had all these leftovers that I didn't know what to do with.