That's not all you need to know, but it's all that's written here.
"Efficiency had been forced on Earth with the increasing population. Two billion people, three billion, even five billion could be supported by the planet by progressively lowering the standard of living. When the population reaches eight billion, however, semi-starvation becomes too much like the real thing...
Think of the inefficiency of a hundred thousand houses for a hundred thousand families as compared with a hundred-thousand-unit Section; a book collection in each house as compared with a Section book-film concentrate; independent video for each family as compared with video-piping-systems. For that matter, take the simple folly of endless duplication of kitchens as compared with the thoroughly efficient diners made possible by City culture.
More and more the villages, towns, and "cities" of earth died and were swallowed by the Cities. Each City became a semi-autonomous unit, economically all but self-sufficient. It could roof itself in, gird itself about, burrow itself under. It became a steel cave; a tremendous, self-contained cave of steel and concrete."
~ Excerpted from Isaac Asimov's The Caves Of Steel