For the US map, the country is broken into counties. For cities, every block within the city limits is shown. In both types of maps, the height is proportional to the number of people per square mile.
Cities with a population of 20,000 or more are available to visualize.
Overall, the visualizations highlight how 4 out of 5 Americans live in an urban area.
Some would prefer the height be proportional to population. Doing so points out how LA county has the highest population in the country with 9.8 million people. Others would prefer the height be proportional to population density so that the volume is proportional to the population. Doing so points out how densely populated New York County (Manhattan) is compared to the rest of the country - some cities build up instead of out. New York County has a population of over 2.75 million in only 23 square miles. Even the 5 counties that make up the 5 boroughs only have a land area of 300 square miles compared to LA county's 4,000 square miles, but they have a population of 8.175 million. With such a small land area and huge population, the population density in the 5 boroughs is more than 10 times that of LA county. To each his own. Choose your metric above to better understand both.
Source: 2010 census