I think you're using an arbitrary metric to gauge how important / populous Boston is. Boston city proper population doesn't rank that highly simply because its city limits don't encompass a large land area. This is more an artifact of how the area chooses to govern itself, not indicative of how large, important, or successful the Boston area is or useful in determining how large a "City" is aside from the # of people governed by a given municipality.
To use a made up example, lets compare two areas of human settlement. Place A has 1.5 million people and place B has 1 million people. Place A has a central city of 500,000 "The City of A." The remainder of the population lives in surrounding municipalities. Place B has one municipality which covers the entire area, call it "The City of B," with a population of 1 million people. Would you really say place B is more populous than place A?
As a more real world example, I've seen articles saying Houston will soon be "bigger" than Chicago as Houston will likely pass Chicago in city limits population. But... that's incredibly misleading / odd metric given 1.) The City of Chicago is about 240 square miles and the City of Houston is over 600 square miles and 2.) The City of Chicago is not surrounded by corn fields.
Boston's Metropolitan Statistical Area (MSA) which includes suburbs / generally measures the population of the continuous urban area is 11th in the country by population and probably higher than that by economic output. So... I'd argue Boston is already top 10 if you use a non-arbitrary measurement system.
en.wikipedia.org
