"The graph and tables on this page attempt to show how the urban hierarchy of the United States has developed over time. The statistic used here is the population of the metropolitan area (contiguous urbanized area surrounding a central city), not the population of an individual city. Metropolitan area population is much more useful than city population as an indicator of the size and importance of a city, since the official boundaries of a city are usually arbitrary and often do not include vast suburban areas. For example, in 2000 San Antonio was the 10th largest city in the U.S., larger than Boston or San Francisco, but its Metro Area was only ranked about 30th. The same thing was happening even back in 1790: New York was the biggest single city, but Philadelphia plus its suburbs of Northern Liberties and Southwark made it the biggest metro area."
Via Seth Dixon, Mike Busarello's Digital Storybooks