Which city can claim the title “biggest city in the world” is somewhat controversial. Some city-like regions are technically separated in to smaller communities; others include suburbs, so which one is it? In order to compare sizes it is necessary to find a common denominator. We do actually have a perfect one. The technical term is: metropolitan area.
The biggest city in the world based on this term is Tokyo in Japan. Approximately 34 million people live in the metropolitan area of Tokyo.
Most rankings name Mexico City as the second biggest city. A staggering 25 million people live in the Mexican capital. Other rankings have Sudogwon (Seoul) as the number two. It depends a little on how you define it. Here we chose Mexico. Then there are a number of cities with over 15 million people; namely Bombay, Jakarta, Shanghai, Sao Paulo, Manila and New York.
Another 25 cities worldwide have a population of about 10 million people, 270 qualify as megacity (at least 1 million inhabitants).
It is estimated that Tokyo won’t be the biggest city in the world for much longer. The Japanese population in general is not growing anymore (rather the opposite) and there is little to no immigration. Tokyo most likely will be affected by the decline. In 10 to 20 years it will probably lose the title to Mexico City. That said; Bombay, Jakarta and Kairo are growing rapidly. In China we can see an ambition to develop gigantic metropolitan areas. Chinese authorities want to turn the area around Shenzen into the biggest metropolitan area in the world with an estimated population of 42 million people. The race for the biggest city in the world is very much open one could say.