To clear any confusion, smaller cities like South Portland and Aurburn are inferior to their respective cities, Portland and Lewiston, and are in the same metro area. Neither South Portland or Auburn have populations large enough (over 50,000 according to the census) to be considered their own separate city, in the classification system I’m proposing. They should be marked as towns, as they already are. However, if two cities in the same metro, like Boston and Cambridge, have populations over 50,000 (both of them do, obviously) then they should both be marked as separate cities. Over 100,000 people live in Cambridge, it is just too large to be marked as a town. 100,000 is the cap used by the OSM display system to note larger cities on the map. 50,000 is a good number, very few cities in New England are that large, and its a number that comes from the US census classification for a metro area. Also, as I said before, the other US census classification for urban areas that should be used for towns is 5,000, and there’s something fitting about 50,000 and 5,000 being used as benchmarks, with the other metros and micros below 50,000, such as Augusta and Bangor, still remaining as cities, as they should.
Also, to note what @ElliottPlack commented on, there simply needs to be a clearly defined exception for state capitals, to compensate for the Montpelier/Barre situation. Montpelier is a rare exception. It is tiny, and would be an insignificant small town in Vermont if not for its status as the state capital. It is so small, that its micropolitan area is not even called the Barre-Montpelier Micropolitan area (it is simply the Barre Micropolitan area). An exception should be made because it’s the capital.
It seems that somewhere in the OSM display system, capital cities already get some sort of precedent. If you zoom out, you’ll notice that the Montpelier label covers up the Barre label. Normally, population determines this, and yet even though Barre is bigger, Montpelier covers it up when you zoom out further. Since the exception for state capitals already exists in the OSM system somewhere, there is no reason not to keep it.