I recently stayed in an AirBnB studio in Paris smaller than 9m². The toilet was shared among the five studios on the same floor.

So it was a private room?

Well OK, but that goes somewhat against my parent comment who was trying to show that things were worse then!

At least in Vienna and Berlin a lot of modern small 1-2 BR apartments are the same apartments that 100+ years ago housed a whole family (or 2!).

For example we (2 adults + 1 toddler) live in an old 70m^2 (750 square feet) apartment that's built in a way suggesting it originally housed 2 families in the 2 large rooms (what we now use as 1 bedroom and 1 living-room) and shared the kitchen/bathroom. So it is very likely 8+ people lived here, and it is now considered small for the 3 of us.

The apartment building is from either the late 19th century or early 20th century.

I imagine a lot of these tiny "single-person" apartments in Paris used to house a lot more than 1 person 100+ years ago.