Wouldn't one normally in real world designs just cut the roof out to get more height clearance for the floor below for the particular narrow parts? Then one could install taller facilities in the deck below after all. I'll grant you that isn't in the depicted picture, but still if we assumed a sane designer you'd think they'd not just leave empty space if they could find a use for it, be it machinery or modification of the deck layout. As such I'm not entirely sure we should take the pic as the final gospel truth of everything and that if nothing is displayed in it for a section that means there is nothing, that might be expecting the QM to do put more detail in to things then they may really want to.
So I guess in the end the question is a bit like... is the design just all direct game logic, or is there a certain level of fluff to it? I guess?
It actually was also built with heavy shields though. So if the tonnage range doesn't differ much then in pure toughness at least there wouldn't be a big difference. Thus why some people can wonder if there is a point to the other class if they're to close, because you could just create a spin-off class like they did with the Attenbourough instead. In which case you'd gain some more efficiency in having more of the same designs where parts are easy to supply of.
Still this is all predicated on the idea the tonnages would actually end up close. Like if this Kea class only ended up at 200.000 ton or 210.000 tons.