In a previous post, I noted that the top-75 U.S. cities' share of the total population peaked in 1940. This makes sense because people have been suburbanizing beyond (almost) fixed city boundaries for many years.
The Census Bureau also tracks population by urbanized areas which are not defined by political boundaries but which follow actual development -- "where the lights start when you fly in at night."
And the amazing Wendell Cox makes it easy for everyone to track and compare the top 33 UAs since 1950. Their share of the total population has grown over the last 50 years -- from 33% to 40%.
Urban economists rely on agglomeration economies, the all-purpose and all-manner of "glue" that binds all sorts of activities together. Interestingly, these effects have not gone away, even as communications costs have plummeted. Rather, they have become available over larger areas. The "death of distance" has been overrated.