20 is maybe going too far, but a bigger space is often just harder to keep clean, especially with kids. Of course if you're raising kids in a city apartment you'll want them spending a lot of time out of the house (whether that's parks, community centers, just shopping, ...) but that's a good healthy practice anyway.
How practical is raising kids in a 20m2 studio? Especially if you've got multiple. I'd bet not nearly as practical as in a house. Plus the recent trend of building lots of smaller apartments instead of a bit bigger and way more practical ones.