For business planning purposes, it is preferable to assess the total size of a conurbation rather than the official quoted population of a "city". City boundaries have in many cases been significantly outgrown in the years since lines were drawn on the map. This results in unreliability of data based solely upon population figures falling within a political or historic boundary. The example of Manchester provided by JES is an excellent case in point. Here is an urban centre which has experienced meteoric growth since the industrial revolution the early 1800's; a growth which political boundaries have never closely reflected.
If you are planning an enterprise such as those which interest us on PPRUNE, the only useful data is the population of the entire conurbation. The arbitrary figure representing the people who happen to live within a long outgrown boundary line is more of a hindrance than a help to accurate planning.