Against an aggressive opponent Rab would almost certainly have led out for something like 1/4 or 1/3 of the pot on the flop with JJJ hoping I would flop something big enough (or decide to make a move) to raise him.
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so why exactly does he not do this with a set of twos?
He does, on the turn when he hits his set and bets about 1/3 of the pot.