If you add the fact that there are many dungeons to visit AFTER you have won the game... I'd say it is much bigger overall.
I'd say the playability after the game has ended is pretty low honestly, Grim. Unless you are into a pure hack and slash game with little or no challenge, which I really am not.
I didn't complete most of the Battlefield sections in the actual game, as they really aren't my thing. They are a great idea, don't get me wrong, but if I want a game wholly like that I'll fire up Diablo 2 which does it a whole lot better...