That has nothing to do with the actual size of Boston. That's a political artifact more than anything else, as neighborhoods around Boston didn't want to get annexed around the turn of the century. Boston's continuous urban population, which includes Cambridge, Brookline, etc. is over 4 million.
Having lived in Atlanta, Boston is a much bigger city despite Atlanta having a population of 4 million within it's proper city limits. That's why you never use city population over metro area or continuous urban population statistics to determine actual city size.
4 million? That seems like a lot. As a New Yorker formally from Boston, I tend to think of the city as the places that the subway can get you. Coming from wikipedia:
Actually, there's quite a few, if you you by what you get get to via the subway. That means Everett, Braintree, Quincy, Newton, Medford and several others.
But there's a more systematic way city size is measured: MSA and CSA. Boston's MSA ( http://en.wikipedia.org/wiki/Boston-Cambridge-Quincy,_MA-NH_... ) is 4.5 million, with 4 million being defined as "urban" population. These are all places served by MBTA (which claims to serve 5 million).
Now, the main issue in the way you're looking at it is that those of us from the northeast have an entirely different view of what "urban" is. There are places in the center of Atlanta that have laughable density compared to any northern city, and the Atlanta subway system serves maybe half a million people. And yet Atlanta has a metro population of 5 million with an urban population of 4 million. The threshold for urban is very low compared to what we're used to.
The truth is that the vast majority of American cities feel like they're mostly suburban sprawl. Yes, Boston is dwarfed in size by NYC, LA and Chicago, but Boston is also roughly the same size as SF, DC and Miami. And no matter what the statistics of city-limits population say, it's has more people and is more densely populated than most other cities in the country, including San Diego, Minneapolis, Charlotte, Detroit and Seattle.
As I recall, it's the 5th largest CSA, 10th largest MSA and 4th most densely populated city in the country.
Yeah, it probably has to do with what we consider urban, or feel to be urban.
The MSA for New York includes places no one would ever consider parts of NYC -- if I'm out on long island or in connecticut I'm not in the city. Just like if you're out in the Boston suburbs you're not really in Boston/Cambridge.
If it really is technically 4 million that's cool, but when I'm in Boston it doesn't feel like a city of 4 million. Just like NYC doesn't feel like a city of 19 million, even if that's what its MSA is.
Boston feels like a small place, just like SF. Nothing wrong with that, it just is what it is.
Having lived in Atlanta, Boston is a much bigger city despite Atlanta having a population of 4 million within it's proper city limits. That's why you never use city population over metro area or continuous urban population statistics to determine actual city size.