Yes but USA is less dense than France and a lot less than England.
USA the towns around New York was created with the development of New York city.
But in England or France cities like Oxford or Chartres was not created and developed with the development of London or Paris.
I think that Metro area concept should change by countries or we can said that Brussel metro has around 30 million inhabitants.
For my London metro has 14 million inhabitants and Paris metro 12 million inhabitants even with a smaller population than others megapolis, London and Paris are two of four most importants cities on the earth.
Paris or London metro would be the 16th largest economy on the planet. (between Netherlands and Australia)
Show that in this site the most visited cities are London and Paris.
http://www.mapsofworld.com/cities/