European cities in the 1600s were massively over-populated for much of the same reasons non-Western cities are massively over-populated now: The rise of industr

European cities in the 1600s were massively over-populated for much of the same reasons non-Western cities are massively over-populated now: The rise of industrialization, a lack of available housing, poor infrastructure, and massive population growth coupled with rapid urbanization of a traditionally agricultural workforce. London, as just one example, was an ugly, disgusting, miserable place until the mid 20th century, so it takes time to sort these things out. Cities in India might be a mess today but once their population growth simmers down and the cities can get better organized they'll be much less crowded. As filthy and disgusting as the slums of some cities in India are, London was worse. The Thames was so polluted with sewage that paddle boats would churn it into this vile froth, like bath bubbles from hell. People who fell overboard from boats would drown because they couldn't be seen through all the foam. Even though the Parliament buildings are right on the river, and the smell in the summer was absolutely unbearable, it took decades for a proper sanitation system to be built because of a lack of political will. Better infrastructure means you can even out the population density better. India, like many other industrializing countries, is way behind the curve. China is one of the few that's been building infrastructure like its going out of style. It's not the one child policy, but aggressive city engineering that's keeping a lid on the problem.