This is my take on a Japanese Modernist ultra-compact. I took inspiration for the design from the work of Atelier Tekuto's Yasuhiro Yamashita, who is the absolute god of designing buildings that feel immense on the inside, but are truly tiny on the outside. This particular work is mostly inspired by his "Penguin House", although not a replica by any stretch.
Would I be correct in guessing that ceiling height has a lot to do with the feeling of spaciousness?
Correct. It's a number of factors, but ceiling height is a big one. Sunlight being let in without any actual massive window walls is another, as having too much view of the outside actually makes the interior feel smaller by comparison. The solid windowless outer walls allow it to feel private, secure, and home-like, while simultaneously denying one a view of the outside world for scale.
The reading room in particular feels massive without a high ceiling because rather than walls, it has railings out to the "wings" - even though one cannot occupy that extra space, or really get a good angle out the windows, the sunlight and openness make the room feel positively massive, for being wedged into the center of such a tiny building.
It does look tiny outside, and the interior is indeed wide. Aside from ceiling height, I guess the use of glasses also contributed to the interiors' spaciousness, since you can directly see the view from the outside. You should have posted an image of your inspiration for this one.