Quote Originally Posted by Tavrobel
It's not a fact because we don't have empirical evidence comparing the universes.
But we can compare the strength of the enemies that they've beaten.

Sora and Riku managed to beat Xemnas with the power of KH. They beat the most powerful character in the KHverse (confirmed by Nomura), who was further powered up with the ultimate power source of the KHverse.

The various FF characters had to assemble teams of 4-14 people to beat their respective final bosses. And most of the final bosses didn't have anything close to omnipotence like Xemnas did. (Kefka was probably the closest to omnipotence with his "controlling all magic minus the Espers" schtick, but it took a party of ~14 FF characters to take him out)

There's only one solution: put Sora into the next installment of Smash Bros.
Well if Solid Snake could...