l wouldn't say it was small. You have Dragons, Cyclopes, Big ass birds, Big ass Beholders, Hydras, Giant Lizardmen, Chimeras and Ogres. That's more big brutes than a lot of fantasy open worlds have these days without even counting the regular enemies or the expansion ones.
It would be nice to see how the game would've let us fare with something Shadow of the Colossus sized, like giants, earth elementals, leviathans, behemoths and renanims, but the final boss in DD did deliver on that and I'm content. So I don't see how it wasn't fully explored. Its honestly everything I ever wanted in a game in every area except story and I still say all studios should learn from it.