There was an architect, Le Corbusier. He was a socialist, so his projects of future cities involved a lot of public spaces where people spend their free and working time, while a person’s home was just a small area for sleeping and eating breakfast. The Soviets took the idea of small personal homes, and dropped the “nice public areas” part.
It’s cold in the winter because the walls are quite thin
You can hear your neighbours loudly speaking
I was lucky to have a normal-sized room in a later “Brezhnevka” house, but many of my classmates had rooms (if they had a separate room at all) where you had a bed, a cupboard+desk combo, and a chair in the middle, that you have to remove to get to the window. Japan-sized stuff.
Speaking of Le Corbusier, as his main (I know, that’s subjective) achievement was a technical approach to ergonomics - all sizes in his projects were based on human sizes and proportions. Meaning that a height of a ceiling is a height of an average adult man raising his hands, + some space. It worked, and it’s cost-effective, but you really like some extra space, and have more than 3 sq.m. toilet.