It's going to be smaller for one obvious, and several less obvious reasons.
Obvious reason being that it's just the initial release versus a game that had three expansions.
Less obvious reason is populating the world. Original WoW was a desert. You could go for a while without seeing anything of particular interest. Perhaps the best example of this is issue the zone of Silithus.
On the other hand, from what we've seen GW2 took the same approach that blizzard took in TBC, WotLK and Cataclysm. Smaller sized, but densely populated world.