Whilst it's common sense that a bigger family should be allowed a bigger home/more bedrooms, it's not always as black and white as that.
As has been said, there are too many variables. EG, I know several couples who live in 2 bed bungalows owned by a housing association, and have lived there for for 20-25 years. There have been several comments from a couple of people who have a family member on the housing list, who have said, 'you could put a family of 4 in those bungalows.'
Why the F should they move out of their home? The home they have been in for 25 years? And where are they supposed to go? To a shitty council 1-bed? These particular 2 beds were built for private sale in the 1970s, and the builder went bust, so social housing took them over. They are roomy and lovely, and have big gardens, driveways, and garages.. And they look nice. ALL the 1 bed bungalows in our area are like shoeboxes. No room in them hardly, no storage, no parking space, no garage, and a 10 foot long front 'garden,' that has a public footpath so close to the house that people can see straight into their windows.
As one of them said, just because someone decides to pop out a couple of sprogs, I am meant to sacrifice my home? Not likely!