Graph of Boston's historical population, and its population's rank among US cities, over time. Population peaked in 1950, and the rank has been ever-increasing: we were the second- or third-largest city in 1800, and now we're the 20th-largest or so.
(Source: raw data, gathered from the Census Bureau)

In fairness: if you look at the population of Metropolitan Statistical Areas, the Boston metro area is 10th rather than in the 20s. The metro area stretches west to Worcester, north to southern New Hampshire, and south to Rhode Island, last I checked.

Boston should be proud that it’s growing at a healthy clip (healthier than the Commonwealth’s growth rate as a whole). Personally, I’ll be a lot happier when the suburbs shrink and the city grows.