The reason betting 200 here is a good idea is that if you bet more, a good draw may push into you, forcing you to make a decision for your chips as to whether your TPGK is good vs his possible draw/set/two pair/perhaps even better kicker. Betting 200 he is more inclined to call on the flop, and when he calls the chance of him completing his draw on the turn is much lower than the chance of him -not- completing it, and when you put the real pressure on him on the turn, he will be more inclined to give up on it. If his draw connects, you can get away from the hand much easier, and it was likely that if he (anyone) had a good draw they weren't going to give up on it either way, with a big bet from you inducing a push from them. A bet of 200 here will also take the pot down just as often as a somewhat higher bet if no one has anything worth continuing with. This is once again a spot where controlling the pot ends up making you more chips while keeping it easier for you to get away from the hand.