Thanks for replies, and back to my original question. We've just got back from a 4,000 km camping trip to Norway in the Jazz, and we've decided to keep it.
A few more test drives since my OP (plus sitting in a Model Y at a campsite) convinced me the step up to an SUV buys almost nothing inside. The HR-V makes the point best with its 319 litres of boot against the Jazz's 304 on the same platform. When I sat in one it felt like a lifted Jazz. Whatever the extra exterior width is doing it isn't reaching the occupants.
Sometime last month after the dealer I also became fixated on centre consoles. Front seats are pretty much the same width in every car, so console width works as a good proxy I reckon for how much of the exterior width actually makes it into the cabin. I peered into many parked cars and eyeballed interiors, which I suspect looked like I was about to steal them. It's a rabbit hole, most of what's down there turns out to be door thickness.
On ride comfort, I'd agree with what was said above, it's suspension tuning (materials, geometry, etc.) rather than size. Anybody who has ridden aluminium and steel frames will know what I mean. The Jazz seems to have been tuned to feel more "sporty".
The gap in my reasoning in the OP is the roof box: it's less about boot litres and more about not wanting a box up there. That's what will eventually push us to a proper MPV rather than to anything in between. We saw a lot of Kia PV5s in Norway and liked how they looked. It's a shame we don't get something like an Odyssey or Stepwgn here in EU...
We'd need only one car either way, since the metro is a minute away and neither of our commutes needs driving.
In the jazz boot I managed to fit a huge stroller chassis (Graco evo XT, just the chassis) + a small one (yoyo babyzen) + backpacks etc. All in the boot. I could also have a lot of space for putting a changing mat + kid for changing, in the Jazz boot lol. The boot is amazing.
sebastiand, that's roughly our experience too, though the chassis we have behaves differently. Worth its own thread maybe?