No, sorry, this only further confuses. First of all neither Worcester nor Manchester are in the Boston MSA, so they're not great examples. They are in the large

No, sorry, this only further confuses. First of all neither Worcester nor Manchester are in the Boston MSA, so they're not great examples. They are in the larger designation -- the CSA -- which includes cities as far afield as Providence. The whole reason CSAs exist is to help us make comparisons that do include cities that far outside of the core urban area. Unfortunately for us, the OMB hasn't defined a CSA for San Antonio-Austin, so it's hard to compare here, but almost certainly the CSA of Boston-Worcester-Providence would still be larger than San Antonio-Austin. The whole reason the OMB would define a CSA for San Antonio-Austin is to let us make these kinds of comparisons in a fair way without having to guess where San Antonio ends and Austin begins. (I wish they would!) Additionally, these are not arbitrary designations. The whole point of MSAs and CSAs is that the OMB is trying to come up with reasonable ways to compare regions that aren't arbitrary. (I mean, the conditions they decide are most important are arbitrary in the sense that they could choose some other conditions, but that's kind of a silly way to say that it's arbitrary. What slurps up what, in other words, is well defined.) All of this is just to say that there are many ways to compare regions and, again, the worst of all of them is using political borders. There is a movement afoot in St. Louis to combine the City of St. Louis and its surrounding County. Nothing about "St. Louis" actually changes the day after that measure passes (if it ever does), but St. Louis would jump way up in city rankings. The results of an 1876 law that set St. Louis's borders where they remain today is a very silly way to decide how big "St. Louis" is. Edit: Another way to say this is that using political borders is like resolving the question by taking a vote. Which city is biggest? Let's just let the people vote! MSAs and CSAs exist to help us make better comparisons than that. Edit: yet another way to say this is that your follow-up comment notices that it's hard to make comparisons because it's hard to know where one thing ends and another thing begins and which things should "count" in comparisons. That's the whole question and it's why we should use MSAs and CSAs. They're at least trying to answer those questions in a way political borders don't even address.