The biggest capital in the world is Manila on the Philippines. That of course is if you only look at the actual city and not the agglomeration. The second biggest capital by that definition is Moscow in Russia followed by Dhaka in Bangladesh.
Counting not just the city itself but the agglomeration, that pictures changes dramatically and lets face it looks a lot more familiar. The biggest city in the world, looking at the metropolitan area is and not surprisingly Tokyo. The Japanese capital is also the biggest city in the world.>
The second biggest capital in the world is Mexico City closely followed by Seoul in South Korea. These three are also listed as the three "Biggest cities in the world".
The biggest capitals listed by continent are:
* Africa: Cairo -> Egypt
* Asia: Tokyo -> Japan
* Europe: Moscow -> Russia
* North America: Mexico City -> Mexico
* Oceania: Wellington -> New Zealand
* South America: Buenos Aires -> Argentina