One more thing, Fuel, Daggerfall, and Just Cause 2 are all procedurally generated, meaning it's the same textures and terrain, but in different sizes and places. Xenoblade X and Zelda U are both handcrafted, and therefore more detailed and natural, a truly open world.
For anyone wondering, this is incorrect. 400 sq km is actually about 150 sq miles. Also, Xenoblade couldn't have been more than 30 sq miles, and is very likely quite a bit less. I actually made an account just to say that. XD it was bothering me that someone was spreading so much misinformation to so many people. It's still pretty big nonetheless.