The following graphic from @Amazing_Maps [2]struck us in its stunning comparison between Greater London and the Greater Tokyo Area superimposed over England.
With a population of around 36 million, making it by far the world's most populous metropolitan area, it covers an area of approximately 13,500 km² (5,200 mi²), giving it a population density of 2,642 person/km² - which is more than twice the population density of Bangladesh.
The area has the largest metropolitan economy in the world, with a total GDP (nominal) of approximately US$1.9 trillion (¥165 trillion) in 2008.
And visually:
[3]
Souce: Amazing Maps [2] [2]