Higher density happens because investors want to maximize the use of land in desirable areas, not because land is unavailable or difficult to zone. I live in Bucharest, a city of incredibly high RE development, and most of the new buildings are more compressed and crowded in on each other than they were under communism, which is a common occurrence in the former Soviet satellite states [1].
From my window, I can see across the Delta Vacaresti, a wildlife reserve that was once a neighborhood that Ceausescu just had time to raze to the ground for some new insane scheme before his people shot him. Across the way, I see three massive multistory residential blocks going up, crowded together like pedestrians on a metro train at rush hour - only one side of the front-most block has any view of the Delta, while the others literally live in each other's shadows, lacking sunlight.
By contrast, communist design principles designed a whole neighborhood, with schools and hospitals and doctors, with the 'megablocks' seen as small cities in themselves that needed dedicated support systems.