Also, market size is not everything. San Antonio is one of the smallest markets in the NBA, but they clearly have an advantage due to their recent and sustained success as a franchise. That success has raised their market value, in that surely NBA fans outside the San Antonio area want to watch the Spurs play on television.
Miami, despite the city's glitz and glamour, is not really a large market either. For obvious reasons, they have not had much trouble acquiring talented players, though.
My point is that we can get good players to come to Boston. But, like with every franchise, other than possibly the Lakers and the Knicks (and maybe Brooklyn and the Clippers), it will take good management to make the team an attractive destination.