Density over scale, always. The world may be big, but how much of it is composed of fields with nothing much to do? How many enemy types are there? Characters? Sidequests?

....but not bigger than GTAV's and won't be bigger than FFXV's.

Question to your 1st question: Does that really matter if just exploring the world happens to be rewarding and fun in and of itself?

Answer to your question: Dozens, if not hundreds. Something like 400 different missions total.

Also one clear point of differentiation: 100 hrs into many RPGs, you're always OP compared to every enemy in the game which makes carrying on very dull. In contrast XCX almost always has more challenging enemies to face even as an advanced player.

XCX has a much bigger emphasis on exploration and combat whereas many RPGs, even large open world ones, focus on being easy to traverse and relatively simple combat systems

Nope. XCX at 400 square km or 154 sq miles is bigger than GTAV's 100 sq miles

But the previous world was much smaler, it wasnt trying to brag about being super hughe.

What does that even mean? 

All sensible questions, I think space has its own rewards, beauty and atmosphere, in real life we have vast fields, oceans, deserts, mountains, skies that have an amazing positive effect on our human psyche

That it's bigger than I though it would be on the Wii-U?

My oh my. I sure hope there is a system in the game that resticts the amount of time you can stay in a Skell to avoid this happening...

From what I gather fuel is costly, so you want to save your Skell for only enemies that are a pain and you'll need it to beat. But I can be wrong about this.