I understand, but its neat how close they stay to the original source material by only giving them their Ultimate Weapons. Though I don't remember Lionheart being that big...

From what I've read, they also follow the rules of their games so despite not looking like much, Squall and Tidus' weapons actually give them a bigger boost than one would think. Sephiroth's is disappointing but the Dissidia team just couldn't get Safer Sephiroth to work. I believe the director mentioned Safer Sephiroth could only have been implemented properly by scaling him down which ended up making him look weird cause suddenly he shrank.

It would have been nice though as I am not very fond of KH Sephiroth form.