Metro area is the most arbitrary and also unfair measure to use though. It's arbitrary because it isn't defined by an actual political or economic boundaries, and it's unfair because it weights towards large centralized cities with mass transit leading in and out (it's part of the damn definition of a metropolitan area in fact).
Yeah, why on EARTH would you want to use a definition based on commuter area when discussing mass transit and commuting?!?
Um... Because cities which have built up larger mass transit systems will tend to therefore encompass more people within that mass transit system. It's circular to then argue that because the largest metro areas all use mass transit systems that this means that mass transit is "better". Um... They are larger because they have more mass transit! It doesn't tell us anything about whether they are "better" in any way. Just that more people are connected via that metro system.
Counties which do not utilize as much mass transit (especially trains and subways) will tend to be broken up into more metro areas because each city within the county is more likely to run its own bus system. That's why San Diego can be the 8th largest city, and the 6th largest county, but only the 17th largest metro area (all by population).
The Wiki page uses the OMB definitions of metro area, by the way. You know, those areas without any non-arbitrary boundaries and stuff...
Which is defined as an area around a city with a population of at least 50k which is serviced by a single common set of transportation systems. Surely, you can grasp how areas which have spent lots of money connecting the city center and all the surrounding areas by train are going to tend to therefore include all those surrounding areas in their corresponding metro-area, while cities which do not do this will not? Or is that difference between San Diego's population ranking as a city and county versus metro area just an unsolved mystery to you?
It's like your brain cells are firing, but no information is moving around.