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