They could be small partly because several of the feature of modern homes that are regarded as indispensable now were mostly absent from such dwellings: kitchens, bathrooms, and toilets in particular.
I have an ex-colleague who lives with his wife in Oslo in a 45 sq. m. flat but that does include a bathroom, kitchen, and a bedroom.
That would feel claustrophobic to me having lived for the last 34 years in a house with three times the floor area, 800 sq. m. of garden, and a cellar. Nonetheless the demand for apartments where he lives is such that his 45 sq. m. apartment is worth almost double what my 130 sq. m. house and garden are worth.
We might not need much but many of us feel much more comfortable and happier with more!
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.
What you say and add to my comment is pretty much the case. I used to live in a flat in Drammen, much bigger though, about 72 sq. m. We had a storage area in the loft of the building and as you say there were communal washing machines. But in fact we would have had plenty of space to have our own washer and dryer.
I would make one point though:
> the two countries are sufficiently similar
That's true for most of this discussion but Norwegians until quite recently almost overwhelmingly wanted to own their own their home (over 80% do) and prefer a detached house, most still do. In 2010 according to SSB half of the just over two million dwellings in Norway were detached houses (enebolig). My impression of Sweden when visiting places like Gothenburg, Västerås, Linköping, Ludvika, etc., on business and holiday is that Swedes are noticeably more likely to live in flats. My house is pretty much in the middle of the size distribution for detached houses (see https://www.ssb.no/bygg-bolig-og-eiendom/faktaside/bolig)
48.1% / 1,198,659 households live in single family houses ("Enebolig"). The Swedish "småhus" total is 44.9%.
47% / 1,170,000 or so live in multi-family housing ("Tomannsbolig" / "Rekkehus, kjedehus, andre småhus" / "Boligblokk"), compared to 51.6% in Sweden.
4.9% / 121,486 live in "other", which I'll map to the 3.6% "specialbostad" in Sweden.
This matches your observation that more Swedes tend to live in multi-family homes compared to Norwegians, though I don't think it's a substantial difference. (I hope I did my numbers correct!)
FWIW, my impression of visiting Oslo is that they mostly live in flats too. grin
I have an ex-colleague who lives with his wife in Oslo in a 45 sq. m. flat but that does include a bathroom, kitchen, and a bedroom.
That would feel claustrophobic to me having lived for the last 34 years in a house with three times the floor area, 800 sq. m. of garden, and a cellar. Nonetheless the demand for apartments where he lives is such that his 45 sq. m. apartment is worth almost double what my 130 sq. m. house and garden are worth.
We might not need much but many of us feel much more comfortable and happier with more!