All it takes to kill Kaido is a small island level attack with some haki on it lol
You're COMPLETELY wrong.
The short answer is :
By comparing the Sunny to the huge lanterns on Onigashima island we can determine that the island is about 20 km large.
It would make it 3.3510322×10¹³ m³.
Since rubber has a density of 1,522 kg/m³ this means that fist of that size would weight :
1.522×10¹⁶ kg
We also saw Luffy easily toy with the electrical discharge created by a lightning, which means that he could move at least at 100,000,000 m/s and yet Kaido could keep up with him.
Giant Luffy could also keep up with Kaido despite his enormous size, seemingly being unaffected by the change of mass.
This would come from the fact that his body has much more freedom as stated by the Gorosei, so he wouldn't have the same problem as Gear 3rd where his attacks were slower.
It's Gear 5th after all.
And even if his fist was still somewhat slowed down, you have to remember that it's not a regular punch, his fist was compressed in order to make it much faster and more powerful.
So we can at least assume that this punch was moving at the same speed as the lightnings Luffy caught which is 100,000,000 m/s.
If you plug those numbers in a relativistic kinetic energy calculator you get that Luffy's punch would have an energy of :
8.31×10³¹ J [ *Small Planetary* ]
That's not even talking about haki, just raw physical power.
So yeah, Yonkos do have the power to destroy the world, they could fight equally with old Whitebeard after all.
Even if Luffy were to JUST let his fist FALL to the ground 3 km below WITHOUT ANY HAKI, reaching the ground with a speed of 242.57 m/s, it would still be Large Island Level at 4.47×10²⁰ J.
Things aren't the way they seem.
Otherwise flat-earthers would be right.
For example reducing Pluto to gravels (which some people call a "Planet") would only require 5.8×10²⁷ J, which is only Multi-Continental and not Large Planetary contrary to what I've seen before.
So next time you want to power scale something, just don't eye-ball it.
Here you're even low-balling the Admirals who are casually Country to Large Country Level with just their DF (no haki amp).