The current playable space for Forge Island are those tiny little rocks in the middle of the ocean. But just LOOK at all of this space that we can't go to and play on unless we glitch outside the map somehow.
Halo 5 needs giant maps like this for Forge. This map alone, if we were able to fully play on it, could probably be the size of 10 Forge Worlds combined.
-
But that would require effort on 343's part.