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...
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...).
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
