Living alone or in small families also means a lot of space for not a lot of people. In Berlin (which has been mentioned above as a very green city) there are also many communities of people that live together despite not being in a family or relationship. Such flatshares/apartment sharing is more ecologically sustainable because one kitchen can serve 3-4 people that would otherwise have to buy their own kitchen and appliances.

Personally, I would not mind sharing a TV (barely use one) or washing machine or garden with more people, but the way western society works in terms of building defaults is biased towards individualism not community building.