Boston is far from a small market NBA team. they have the sixth largest media market in the US and a rabid money spending sports viewership marketplace. They have around the 10th highest ticket price in the NBA and are one of the few NBA teams that holds an equity partnership with their television broadcasting partners.
The Celtics are worth more than 27-28 other NBA teams and has been a regular payer of the luxury tax without it being a problem while they have been contending.
This team is not afraid to spend money to put a winner on the floor and win championships. That can't be said for about 25 or so other teams in the league.
The reason for a lack of free agents is because the Celtics have been over the salary cap for what has been well over a decade acting like a big market team, spending money to put a winner on the floor.If you are spending monet trying to win and constantly in the luxury tax, it means you are not way under the salary cap and hence have no way of signing a big name free agent.
The Celtics have never tried to clear enough cap space to get so far under the cap to sign someone because that means allowing assets to walk in free agency, not resigning your own star players you have developed through the draft and trading away players to get nothing in return.
If you have followed Danny Ainge's management philosophy, that isn't his style. He doesn't let players walk unless they are outpricing themselves and he doesn't trade players away for nothing and he tries like heck to resign his excellent well developed drafted players so that he can get them to play well in Boston or to trade away to get some other asset.
Don't conflate Boston not signing big name free agents recently with not being a big market team because quite honestly, before Steve Nash, the Lakers haven't exactly been signing big name free agents recently either. Bynum, Gasol, Kobe, Shaq, Fisher....none of the Lakers biggest names during the 2000's were signed as free agents and yet, no one is accusing the Lakers of being a small market team.