As the world urbanises and our lifestyles evolve, cities are getting bigger. And I'm not just talking about population sizes. As This Big City kicks off a fortnight of themed posts on Megacities and Microcities, what better way to start than by taking a look at five of the world's 'biggest' cities?

Tokyo – the world's largest metropolitan region

Which city has the biggest population? The answer to that question depends a lot on how you define 'city'. If we look at the complete metropolitan area of a city, Tokyo comes out on top with around 35 million residents. But with the city's metropolitan area being the largest in the world, covering a whopping 5,000+ square miles, the total population might be impressive but the density isn't.

Manila – the world's most dense metropolis

The world's most dense city is located in the Philippines. The city itself (excluding its suburban regions) has a relatively small population of 1.6 million, but with this area covering less than 15 square miles, Manila wins hands down on the urban density scale. 111,000 people per square mile? Impressive stuff.

Nauru – the most overweight city in the world

Which city has the biggest population? Nope, I'm not repeating myself, I'm thinking about which city has the most overweight or obese population. The World Health Organisation states that those with a Body Mass Index (BMI) of 25 and above are 'overweight' and 30 or above are 'obese', and using that definition, Nauru comes out on top with an… enormous… 94% of its population registering as overweight or obese (and yep, the tiny pacific island of Nauru is actually a country, and its de facto capital is Yaren).

La Rinconada – the world's highest city

At 5,100 metres (16,700 feet) above sea level in the Peruvian Andes, La Rinconada is the world's highest city. The small population of 30,000 have no plumbing and no sanitation, though they do have one thing which may explain how a city could become established in such a location – gold. The city's economy is almost entirely dependant on production from the nearby gold mine.

Shanghai – the world's most populated metropolis

With a population of almost 14 million in Shanghai's established city border, China's largest city is also the world's largest city. Obviously, counting only the population of the urban locality without its suburbs is pretty ignorant of Shanghai's true population, but by this definition, China is home to the world's biggest city.

In the end, it all comes down to how you define 'biggest' and how you define 'city'.

This Big City's Megacities/Microcities series runs until May 25th. 

Images courtesy of decade_null, oisa, jdn, sadie and maude's place, John Donaghy and Gregory Jordan on flickr