Who are you to say what is and what isn't necessary?
Yes it does explain it! Oda is adding unnecessary shit to the story in order to extend it as long as possible. What was the point of Luffy taking five chapters to run up the stairs so that he can reach the top at the roof in chapter 1000? That was a MARKETING DECISION. That was NOT a story decision.
You're joking right? Oda has done this with Luffy, literally Every. Single. Arc. Pre-skip, Post-skip, doesn't matter, it happens all the time. Quit acting like this is something new, or acting like other stuff isn't happening in that time frame, like it's just Luffy running up stairs and that's it.
You obviously are having a really hard time understanding what bloated means.
I know what bloated means, and I also know what kind of formula and writing style that I've been subjected to with One Piece since I started reading it weekly over 15 years ago. Oda likes his World to feel alive, and to draw scenes outside of just the main protagonists, and to make sure every single character he introduces every arc to have some importance to the story, no matter how little it is. Again, this is not something new. He's been doing it for decades.
People like to bitch and complain about the "Tama plot", but guess what? It's necessary for the story to make taking down an Emperor of the Sea with 30,000 troops under their command believable. You don't hype up a specific group of characters (The Yonko) for decades, and then have the protagonists run into their territory with a handful of allies and have them magically take one of them down like it's no big deal.
He brought out the full force of the Marines and half the Shichibukai just to take down one of these guys who was sick and dying, but don't worry, throw the Straw Hats a few ninja's, samurais, and Minks, and they're good to go to take two of them down, amiright?
Maybe Oda should just have Luffy go the fairy tale route and him run in there with the power of friendship on his side, or the Naruto route make him so strong he can solo everyone on Onigashima. That'd be much, much better right? Get rid of all that bloated unnecessary fluff that he puts in there to make the story of going up against not one, but two Yonkos believable.
So lets take your example, Dressrosa. Dressrosa was 102 chapters long. And lets take your example of building the grand fleet and compare it to something similar pretime skip which was Impel Down. Impel Down was 25 chapters and it build up a fleet of pirates in order to help with the war in Marine Ford. So lets do some subtracting 102-25 is 77. So what happened in those 77 extra chapters in Dressrosa? This is exactly what I mean by Oda bloating arcs unnecessary.
Two situations, not even remotely comparable. With Impel Down, the common goal between Luffy and everyone else in Impel Down was straight forward and easy. Everyone of those guys wanted to escape the nightmarish prison. There is no legwork there that needs to be done.
That's a hell of a lot different than gathering a group of people on a single island who don't know each other, and establishing a connection between all of them in order to make them allies for life. Again, we are going for believability here.
Again, lets use your example of WCI and compare it to to a similar arc that had similar events like Fishman Island. Fishman Island set up Big Mom as a future antagonist, got Jinbei to join the crew, introduced the Fihsman racism backstory with celestial dragon, provided info the void century via Joy Boy, provided info of Luffy returning back to Fishman ISland where he will end up destorying it, provided Posedion and more and that took all of 50 chapters. WCI was 80 chapters. So lets do some math again, 80-50 = 30. Even though Fishman ISland had way more info dump it still was less than 30 chapters than WCI. You are literally proving my point in your examples.
You starting to get the concept of believability? So then pray tell, how you can compare the Straw Hats as a collective group going into Fishman Island and wrapping that up in 50 chapters, to 4 of the Straw Hats invading a Yonko's territory, and wrapping it up in the same amount of time? WCI already suffers from us as the readers having to massively suspend our disbelief in order for it to have been pulled off despite all the "bloated" (as you put it) content written into it, and you think it could have been done with 30 or less chapters than it already was?
Perhaps you should go read Toriko and see what happens when all the "bloated" content is removed from a story. I love that series, and the entire last half of it ended up being one of the most unsatisfying experiences I've ever had to endure. Wave after wave of nothing but info dump, with no solid build-up for any of it.
If that's the type of story you like to read, then to each his own, but quite acting like you have it all figured out with your constant "IT'S ALL ONE BIG MARKETING PLOY" crusade that we have to sit through week in and week out. The fact of the matter is that Oda isn't doing anything now that he hasn't been doing for decades. The only difference is now is that the stakes are higher so more subsistence is added in order to make the story more believable. I like it. Countless other people like it. You and other people don't. Personal preferences are a bitch.