From a reader:

A…factoid that those following your debate on overpopulation might find interesting. Manhattan is pretty crowded: more than 1.5 million people in about 23 square miles. But Manhattan has had the same square mileage for a while now, and it used to be much more crowded: more than 2.3 million people in the 1920 census. Another way of looking at it, relative to the growth of the rest of the country, is that in 1910, 1 in 40 Americans were crowded into Manhattan. Today it is more like 1 in 200.