We need farmland to feed the population, and personally I think we need some forests and wilderness as well - for recreation, for the sake of the nature itself, ecosystems and biological diversity. Either we need to stack up people (high-rise apartments) or we need to become fewer people. How much area would we need for the population of Beijing if everyone was supposed to have a house with some land around? Not to forget that a sprawled population needs lots of areas for roads as well.