Quote Originally Posted by VeloZer0 View Post
Or Ultima Weapon. For all we know it could only weigh a pound or two, being made of magic and all.

As far as I know Lightning's weapon doesn't have any story significance in the least.
Well, of course it would be anybody's first instinct to equip Cloud with the Ultima Weapon. But that is an absolutely unique thing kin to the crystal sword. In Final Fantasy Tactics, You have to obtain the Materia Blade to make cameo Cloud useful. I have the feeling the Ultima Weapon is also composed of crystallized materia. It probably becomes lighter to wield when the user is at full strength thus the HP@MAX=ATK+ formula.

There were a lot of smaller choice swords in VII, the only reason Cloud had the Buster Sword was because it was a remnant of Zack. But who knows, Cloud could have probably wielded the memory of Zack as a weapon in itself.