Agreed,
16 inches is extremely large for a bluegill. Perhaps you were thinking of redears - that get a bit larger than bluegills.
In regards to size, many things can be a factor. For instance if you keep the bluegill in ideal water conditions with plenty of room and feed large meals regularly, then you will have a larger bluegill in a relatively short amount of time (given you catch it at a failry young age).
And yes, for future refence, sunfish can mean anything from LMB and rockbass to dollar and bluespotted... so size tank is relative.