>The total area of the Pearl River Delta is about 40,000 km^2.

The definition of the urban land for PRD and Tokyo as used by the World Bank report is ~7000 square kilometers (PRD) and ~5500 square kilometers (Tokyo).

They do not count the entirety of either of the two. For the aforementioned areas (that they deemed urban land), they estimated them to have populations of around 42M (PRD) and around 31M (Tokyo).

This area excludes Hong Kong and many other parts.

Grauniad did a very poor job at getting the World Bank report's message across.

Of course larger metros will be...well...larger. I don't think the physical area is meaningful. How about integrated infrastructure, commerce, social groups, etc?

Of course it's meaningful. Because, due to its size, the Pearl River Delta contains what are essentially 11 different cities - each with its own city center. Where as the Greater Tokyo Area includes 6 - all substantially smaller than, and largely centered around, Tokyo.

Speaking about your other attributes... I think the most telling thing about the Pearl River Delta is that 2 of those 11 cities were governed by the UK until 1997 - and you still have to pass through customs and immigration to cross into and out of them...

"each with its own city center" vs "largely centered around Tokyo" is the answer then - Tokyo is cohesive, PRD is not.

The customs thing is interesting. Citation?

Exactly, but that's why the area IS meaningful... Something the size of the Pearl River Delta can't possibly be cohesive enough to qualify as a single metropolitan area (yet). So comparing it to one is a bit arbitrary.

Also, those two cities I was talking about are Hong Kong and Macau. Both were governed by the UK from the end of the First Opium War (1842) until 1997, when control was handed back to China. The terms of the handover included maintaining independent boarders (and forbidding complete freedom of movement and immigration). You can read about it and the other terms of the handover here (http://en.wikipedia.org/wiki/Transfer_of_sovereignty_over_Ho...).

Macau was governed by Portugal and was handed back in 1999. It had been rented by the Portuguese since 1557.

Oh, duh. Yeah. But, still separate customs and immigration.

I don't know about 'citation', but when I crossed from Hong Kong to Shenzhen last month I had to exit Hong Kong's immigration and enter China's and vice-versa on the way back.

It's substantially easier to visit Hong Kong than China (visa at the airport vs. visa in your home country before you travel), unless you have an APEC card of course

You can also buy (180 rmb for most countries) a 5 day visa for Shenzhen at the Luohu border too but the visa is not valid for Guangzhou or Dongguan.