Sure Forge Island has lots of space, but it's kind of useless since about 80% of it is water, and we don't have enough pieces or budget to cover it up. Plus, there's not really any good way to use the water in a land-based map since there's no beaches. Forge Island gives us ten times less budget than Forge World and puts some of the most useful pieces all into one category that only allows twenty pieces at $150 each. The budget wouldn't really be a problem if we had more to work with instead of having to build our own rock walls and such, which Forge World provided.
Forge World also had useful special effects such as purple and orange for different times of day, and Next Gen for a cloudy look (Next Gen in Halo 4 just messes everything up).
I feel that the lack of terrain in the Anvils is actually a very big problem as we only have very little rock pieces if we want t use buildings, walkway larges, Colliseum windows, or round ramps. Plus Forge World had a large indoor palette that didn't get in the way of the natural terrain whatsoever.