The edges of a city's defined borders don't really mean all that much when they bleed into yet more urbanity. The stereotype of sprawling Greater Los Angeles, for example, shows hardly any discernible distinction when crossing over the border from Los Angeles proper to neighboring Inglewood or El Segundo or Long Beach.
So when the city is more than a single city – a metropolis – how do we compare one city or urban agglomeration to the next? For now, we do it very imprecisely, by say, comparing the distinctly dense Hong Kong with radiating London. But a new set of metropolitan definitions is hoping to make city-to-city comparison a lot more sensible in an age of urban agglomerations indifferent to borderlines.