If I have to guess, XenoX will be bigger.
I really can't think of another FF that is as bigger as Xenoblade Chronicles, let alone XenoX which is 5 times bigger.
On the other hand, FFXV seems to be the bigger final fantasy ever (in its world, not in content), so It can be bigger than XenoX
3DS FC : 4339 - 3326 - 7693. Add me :) Nickname Tin