Two superb graphics which highlight how population density can affect the spatial dynamics of countries. The first shows the space, in term of area of US states, required to accommodate the total world population at the same density as in some global cities.
The second (sent by a friend) shows the sizes of different cities and their population. Note the relatively similar populations of New York and New Delhi, and Tokyo and Dhaka, and the manifold difference in their sizes.
This blog has several other interesting similar graphics.