Actually, "metro area" is a misleading term because the size widely varies from city to city. The better comparison is taking a consistent distances for every city so that you you are comparing apples to apples regardless of whether an area choose to roll all the surrounding communities into the city or whether, as in the case of Hartford, West Hartford etc. they chose to maintain separate municipalities.
Distances are different though depending on where you are in the country. There’s more land in the Midwest than the Norheast, ergo cities are built differently.