No. X doesn't have bigger scale. The action is always local with barely anything going on in the background. All you see are mountains (and not much else) in the distance. FF XV has that Leviathan battle, with realistic water, cloth physics, real time lighting effects, with quite a bit of stuff being animated all at the same time (airships attacking Leviathan, while that beast crashes that bridge resulting in the bridge collapsing in real time, etc...). X has nothing on that scale. The overall draw distance is great, but nothing is happening anywhere unless it's close to the player character.
The Wii is incapable of achieving even this:
wii u would run the game without much difference.
I think you're underestimating the tech in Final Fantasy 15. The physics calculations needed for all of those crystal effects around Noctis for instance would be pretty demanding. Every aspect of the game has been improved between the original PS3 version and the next-gen version, hair and clothing physics, lighting, textures, anti-aliasing, etc. The Wii U isn't that much more powerful than the PS3. I see you cited Monolith's X earlier in the article, but X's graphics really aren't that much better than the original PS3 version of Versus 13. If the original code for Versus 13 on PS3 was completed I'm sure that they could easily get it running on Wii U, but I'm not sure that they could easily take 15's code and downgrade everything to run on Wii U. Could it be done, yes, it probably could, but it would take considerable effort, and I'm sure that Square believes the sales potential of a Wii U version doesn't outweight that extra work needed to port it.