I think that's a fair point, but I find your proposed solution lackluster.
Guts or Sephiroth don't spend limited power resources to wield their enormous weapons; they don't enter brief bursts of fury that are the only times when they can fully wield their powers. They can just wield oversized weapon as a matter of fact.
And what is more important, this is not presented as a
tactical decision - not in the sense that there are merits and drawbacks to using oversized weapons, and they're opening certain weaknesses in exchange for the benefits of their enormous reach and damaging power. They wield oversized weapon because they are able to, and being able to do it makes it automatically a superior choice. The Dragonslayer is the only way for a human-scale opponent to go toe to toe with the superpowered Apostles. There is no implication at any point that Cloud or Sephiroth would be just as powerful with smaller weapons, trading damaging power for the speed of a lighter weapon. If you
can wield it, the oversized weapon is optimal.
Now, of course, this isn't always the case. Manga is rife with easily-dispatched mooks and antagonists of the week whose oversized weapons are too cumbersome and sign their downfall. But they're, well, mooks and opponents of the week. And in general, the point is that they
don't have the strength to fully wield these weapons, their blows being slow and telegraphed. When a protagonist is the one saddled with an oversized weapon, it is very often presented as the
best kind of weapon to have - it's just that there is a high enough gate to entry that people using them are rare or even unique.
Obviously this is problematic to implement in an RPG. You want balance, not for every fighter to splurge on max Strength and ISE so they can wield a super-grand-ultra-greatklave and win all fights forever.
But I find the proposed solutions unsatisfying because they betray their source material. Guts, Cloud, Sephiroth, Soulsborne characters - if they can wield their oversized weapon, they just can. They don't need to fuel it with magic, and they are not short-lived glass cannons who must expend huge amounts of power with every blow and be exhausted quickly.
What doesn't betray the source material, then? Well, mostly, the "underwhelming" option as outlined by
@Revlid. Yes, it is something of a cop-out. But ultimately - in Bleach, powerful Shinigami can tank hits from Ichigo with minor injuries. Guts mows through hordes of mooks, but Apostles can take more than one of his blows. FF7 has heavily abstracted combat, but peripheral material shows Cloud having to hit opponents repeatedly, generally without causing gruesome wounds. Bosses can take several hits from an Ultra Greatsword.
Giant weapons are, in most source material, an
equalizer. They serve to put an ostensibly "human" character on a level with their opponents; where ordinary humans are mooks, their giant weapon allows them to deal enough damage that, while they cannot win in a single hit, they can engage the opponents in equal terms. The monster wounds them, they wound the monster. Attrition occurs.
They are, in a sense, Exaltation. They serve both as the visual signifier setting this "human" apart from others, and as the medium through which their abnormal power can be leveraged to deal with opponents that should be out of their league - whether this be giant monsters or supernatural beings of supposedly greater age and power.
Which, in the bottom line, makes it so that Ichigo's ridiculous sword can only deal shallow cuts to a powerful Shinigami, Guts has to hack and slash at an Apostle to kill them, Cloud has to pull off his nine-hit-combo to defeat Sephiroth, etc. So, I'm fine with giant weapon being just "Strength X to wield, +Y damage," because the alternatives are either 1) having them be the optimal choice, 2) having some bizarre system that isn't true to their source material where just hitting with a grand goremaul is functionally equivalent to using a damage-boosting Charms and so costs motes every time.
