Agreed, particularly with Sabaody to Paramount War, they should clearly be one arc because they all run into each other and they're collectively the same length as the other arcs. I assume the wierd small arcs there was a case of Viz suddenly not having hindsight to go off and making a bunch of incorrect assumptions. I think that Zou is small and exposition/setup-heavy enough to be part of Tottoland, though (which I'd personally call "Whole Cake" or "Germa 66").
I think they 'round down' the arcs' volume numbers because they want them to be jumping-on points (when the speed-up happened they had "New Arc! Start here!" stickers on those volumes), but I suppose that means they expected new readers to buy the first "Water Seven" and "Sabaody" volumes and start with the last one or two chapters.
Yeah, I'd have to check the release dates, but I'm pretty sure Sabaody was riiight around when the English volumes actually caught up with the Japanese release (or at least, got within the 10 month window they were allowed to have) and they obviously overcomitted to it, not knowing the bigger picture. The whole New World debacle feels like a blunt force solution to never getting caught out by that again. I get the marketing appeal of going by what starts in the volume so you're always sure to get the beginning of a new arc, but it still just feels so wrong when you know the chapter ratio is 7:1. And it does create its own problems for new readers. Years ago, an ex who was reading blind through my physical volumes was feeling relieved to finally reach the end of Skypiea and ended up confused and frustrated when it went on for almost another whole book after the last volume with its name on the spine
Maybe a compromise could have been an extended recap for the first volume of a new banner if the arc technically started in the previous book, enough to get new readers up to speed without things feeling so awkward for collectors.
If I'd made my edits only a few months ago I probably would have put Zou in together with Tottoland, but I've been rethinking how I look at these stories structurally lately. Zou sets up as much for Wano as it does for Tottoland, and is totally aesthetically and tonally unique from any other arc it might be pushed together with. I think the post timeskip stuff is just fundamentally structured different from the pretimeskip in a way that resists arcs being counted together. The pre-timeskip stuff always had the ultimate end goal of setting up Ace and Marineford, but that was at best a subplot developing independently of the crew's actions, and didn't become apparent how it would come together basically until Thriller Bark. For post-timeskip, the overarching destination of Wano has been clear since Punk Hazard and its 100% the crew's own actions taking them there. Smaller sets of two or three arcs don't make the same satisfyingly closed off sagas they used to because really the whole New World is saga in the pre-timeskip sense of the term. In that respect it was neater to just have them all be named individually rather than trying to give Zou/Tottoland special treatment.
(Names-wise, I never understood how Whole Cake became the most common nomenclature for that particular plot thread. Between Cacao Island, the battles at sea and the mirror world, a huge chunk of the arc is spent off of the eponymous island, so doesn't it make sense for it to be named for the whole archipelago? But that's just me, and things like this are pretty subjective.)