[_delirium]

In many cases I think you'd want a third number, something like the core urbanized area. Metro area is too big, but in some cases city limits are either too big or too small for historical/political reasons. For example Houston includes many of its suburbs, because Texas law is very annexation-friendly. Whereas parts of the urban core of Boston are legally in Brookline and Cambridge, and part of the legal core of Copenhagen is legally in Frederiksberg, but for transit purposes those distinctions aren't that important (Brookline/Cambridge are still part of the "T", and Frederiksberg has a line of the Copenhagen Metro).