The city of Chicago lies entirely within Cook County, and with the exception of a slight overflow into Du Page County, all of its suburban growth lies now and will for some time lie in the same county. Cook County is therefore the second in the United States in population, having 2,400,000 against New York County's nearly 2,800,000. The corporate limits of the city of Chicago include a slightly decreasing proportion of the county's population: in 1890 they contained 92.3%, in 1900 92.4%, and in 1910 90.9% of the total; that is to say, the city's suburban growth is slightly greater than its internal increment. The population of the county now outside of the city numbers 220,000. The area of the city is 191½ square miles; and of the county not quite five times as much, or 933 square miles.

It is of some interest to the problem of government and its possible reorganization to note that only 5 of the 102 counties of Illinois have an area as large as that of Cook, while there are two with actually less area than the present city of Chicago. The average and median size of Illinois counties is 540 square miles against Cook's 933. In population there are only two others that pass 100,000; and the next to Cook, with 2,400,000, or with 220,000 outside of Chicago, is St. Clair with 120,000. On the other hand there are 9 counties with a population of under or approximately 10,000 apiece—less than one sixth of a Chicago ward,—and most of these are decreasing in population.