Okay, now just because Russia is the biggest continent in the entire world, it doesn't mean that it's the strongest in the world.
Ignoring that obvious bolded mistake, you seem to have taken what I said and managed to reverse it and completely change its meaning into something I didn't say.
You say because the universe is bigger, then the enemies must be stronger right?
Not quite. I'm saying that KH's ultimate power source > any FF power source.
Well, in that ENTIRE multiverse, there are only 2 types of enemies!
Humans are enemies as well. Not to mention that these 2 types of enemies are still quite strong, actually.
While Final Fantasy has hundreds of thousands of different type of enemies and derivatives.
Hardly matters, since hundreds of thousands of those enemies aren't even close to being the strongest.
Now, you try and combine and reiterate all of the Final Fantasy worlds into one whole multiverse, and there you have to find the strongest :P.
Doesn't matter. Individually, their strongest power sources and enemies aren't stronger than Xemnas and KH, and putting them together won't make any difference.

By the way, now you're the one trying to cross universes.
Lol, would they? That's because they don't enjoy being right :P
It's actually the opposite, and you'll soon learn that as well.