The destruction caused worldwide earthquakes, how is the "actual destruction" being just island level? This attack surpassed everything what Whitebeard demonstrated and dude is country-continental with his attacks.
It's not about Post Nappa fight Goku and Vegeta, it's about Goku and Piccolo before they fought Radditz.
Well, Alabasta is a country large enough to which you have to travel a few days between villages, so I think Alabasta is decently large. Far larger than DR which has massive landscapes and several towns. Yet, Alabasta is just a small thing in the One Piece globe, the same like Wano. So I'd say that OP world is massively larger because no country like Alabasta would look so freaking small in our globe. I purposely excluded the Alabasta calcs because people rarely accept these but Alabasta is factually large. It was a river which is 50km in width; that's canon information. And the several days travel between villages highly indicates that the country is at least a few hundreds of KM in size.
I mean, look how small Alabasta looks just in bird-view (btw, the Capital is 10km in width):
And keep in mind, Alabasta is just one of the countless islands which exist in the OP world:
The 20 million is most likely just a hyperbole but it puts into perspective how many islands exist so that it's not countable.
When did Oda say OP world is the same like DB's? Source?
Well, the energy being released which caused worldwide earthquakes, thus shifting tectonic plates all around the world, and tsunamis (as I highlighted above, OP world > ours in size) is surely way higher than any supervolcano in our world can pull out, not to mention the sea level rising 1m. I doubt a super volcano rises sea level on 1m. That's some development which we will face in 2100 with bad climate changes.