Image copyright AFP

Which is the biggest city in the world? And why is such a simple question so difficult to answer?

If you search on the internet for the world's biggest city, you'll find various different candidates: Tokyo, Seoul, Chongqing, Shanghai...

Which one you regard as the holder of the title, all depends on what you mean by "city".

Most experts will tell you that Tokyo is the world's largest metropolis, with a population of about 36 million people.

But the core of the city has only eight million people living in it.

The reason it gets into the record books is that the surrounding region - which includes the country's second city Yokohama, as well as 86 other towns and cities - has become so built up that it is now one huge continuous urbanised area.

Yokohama alone has a population of 3.6 million.

Image copyright bbc City proper: Tokyo city was merged with Tokyo Prefecture in 1943 to form a "metropolitan prefecture" - it still contains the 23 wards of the old city

The urban area shown here reflects dense inhabitation (at least 4,000 people per sq km) within administrative boundaries

Defining the borders of a city is no easy task - and there is no international standard to ensure consistency.

Three scholars who in 2009 compared eight different lists of top cities by population, found there were 30 "top 20 cities" in total.

One of them, Richard Greene, associate professor of geography at Northern Illinois University in the United States, says even the most authoritative list, from the UN, "compares apples with pears".

"We tend to think of three concepts of the city," he says.

"One being a municipal definition - the legal city if you will. A second we call the urbanised area, or the physical city - the built-up portion. A third we call the metropolitan area, which some people refer to as the influence of the city - how far out does the city go in terms of its influence, such as commuting patterns."

Austria-sized city?

Most experts rate Tokyo as the world's biggest city because of the size of the population in the larger urbanised area.

Many of the 30 million people who are said to live in the city of Chongqing are actually agricultural workers living in a rural setting

It will probably still be the biggest in 2025, although its population is expected to scarcely increase. The UN expects it to be followed by the two major megacities in India - Delhi and Mumbai, which are projected respectively to have 29 million and 26 million inhabitants respectively by then.

And what about the cities in the country everyone is talking about - China?

Its cities are growing so fast that for the first time more than half the population live in urban areas, it was announced earlier this month.

But China's population statistics can be particularly misleading.

"Virtually overnight, Chongqing has become the largest city not only in China, but in the world," Time Magazine proclaimed in 2005.

But it wasn't true - Chongqing is not the largest city in the world, or even in China.

Why do so many people think it is?

Professor Kam Wing Chan of the University of Washington in the United States, who has made a career out of correcting people's exaggerated claims about Chinese population statistics, explains that what China calls a municipality or city is better understood as a province.

Many of the 30 million people who are said to live in the city of Chongqing are actually agricultural workers living in a rural setting, he says.

In fact, he says, the area is so huge it's about the size of Austria.

'Doom and gloom'

"And if you were to travel from the downtown area to some of the peripheral areas where those 30 million live, it might take a day or two because the road conditions are not that good. So, this cannot be possibly called a city. Because when we call a place a city the general understanding is that we're talking about a commuting zone."

Professor Chan calculates that a more reasonable estimate of the urban population of Chongqing is six or seven million.

The largest city in China is actually Shanghai. It is commonly thought to have a population of 20 million, but Professor Chan thinks 16 million is a better estimate.

He says everyone just loves to think China's cities are bigger than they actually are. He has even had to correct fellow experts at a world conference on global megacities of the future.

"They were trying to paint a really doom and gloom picture of these unmanageable urban giants, megacities with a population range of 20 to 30 million people.

"They were saying China will easily have a few of those in that range, which is not true - they are just picking up on a wrong definition. That gloom and doom scenario will definitely need to be revised."

Richard Greene is co-author, with Richard L Forstall and James B Pick, of Which are the largest? Why lists of major urban areas vary so greatly, published in Tijdschrift voor Economische en Sociale Geografie, 2009