Rent controls increase the average amount of time residents stay within an apartment, concomitantly decreasing the average number of residents per apartment.
The unnatural affordability disincentivised tenants from moving out of their undervalued accommodation. People who would otherwise choose to downscale and pocket the difference would no longer financially benefit from moving into an apartment with fewer rooms.
When apartments are prices according to a free market, people will often be satisfied to share housing with friends or family, saving on rent. Under price controls, a single person will opt to move into an apartment otherwise suitable for three or four people.