How to know Kaido is NOT the strongest character. His "1-on-1" title is a rumor. "WSC" at least has doubt casted on it, and at best, it just means his species are the strongest (i.e., Arlong's species are stronger than Akainu's, but Akainu would fry his ass way past well done). Doflamingo and Big Mom don't wank him anymore than the other Emperors. Has less relevance than other top tiers (i.e., Shanks, Akainu).
Heavier swords amount to higher AP. That was noted by Zoro when saying that Shishui is heavier than his other swords, saying that the air slash it creates is "huge" and pointing out that it even swallows the slashes of his other swords. Is Fujitora's DF perhaps the best powerup for a swordsman? Imagine increasing the gravity around your sword to a point it reaches 100kg or 1 ton.