They do stock smaller trout, as they get what the fishery gives them for that year.
I was fishing a tributary of the Charles river, and caught a 3" brookie.
Now I'm not saying your fish wasn't native (as most trout in most Mass waters are from prior years stocks), but it most likely was the result of successful breeding in that water.
I love getting trout that have had some time to acclimate and breed in their surrounding waters....it makes them less likely to hit the first thing they see, and makes you a better angler by making you try harder!
Good luck and go out and find your own special fishing hole!