> Oslo in a 45 sq. m. flat
While I live in Sweden (about 4 hours away from Oslo by train), I think the two countries are sufficiently similar that I can mention some of my experiences moving here from the US.
Many apartments here have a shared laundry room/area, with washer and dryer, or drying cabinet/room, and place to iron/fold.
I think just about every apartment also has a storage area, either in the basement or the attic. For us this is about 10 sq. m.
Many apartments have access to a common area that can be booked for parties or other larger events. Two rooms with kitchen is pretty common. Some have a bookable sauna, that isn't common. I've also seen places with a bookable guest rooms, which reduces the need for everyone to have a guest bedroom.
Many have some sort of green space with a play area for the kids. Some have an enclosed courtyard where residents can have their own garden. Some have a gazebo, or grill area, and one new construction here has a freestanding shared conservatory (labeled "orangery", but without any orange trees).
We don't have this, but we are a block from a small park and four blocks from a couple good play areas.
Even with these, the result is certainly not as spacious as the house you describe. Rather, I wanted highlight there might be some additional aspects of the Oslo flat which aren't reflected in your summary.
While researching what I wrote I found https://no.wikipedia.org/wiki/Arealberegning_av_bygninger with a chart showing "Size in relation to primary area and number of people in the household - Area standards" ("Størrelse i forhold til primærareal og antall personer i husholdning - Arealnormer"). It puts 45 sq. m. for 2 people as "cramped", with the lower range for "normal" being 55 sq. m. Above 100 sq.m. for 2 people is "spacious" and above 150 sq. m. is "very spacious."
You are right too about kitchen and toilet. In the late 1800s there would have been one shared well, for the apartment building, with a shared toilet. I've read that people would have kept a pig or other animals in the courtyard. And one of the buildings here was the first worker-owned apartment building in town. The historical sign on it says it had a shared kitchen for all the residents.