You generally want to keep the pot small in situations where you'd like to get a cheap showdown. In other words, you think you probably have the best hand, but it is unlikely your opponent will play a large pot with a hand that is worse than yours. KK on a Q9J flop is a good example of this. You can expect to be good, but you can't expect your opponent to put a lot a lot of money in the pot with anything that you beat, except maybe AQ/KQ. If you inflate the pot on the flop, you may find yourself making a decision for all your chips on the turn or river. Checking or making smaller bets early means that bets on all future streets will be smaller. It also makes your opponent's hand easier to read, as he is less likely to bluff or make a loose call in a small pot.

Another time to keep the pot small is in way ahead/way behind situations. For example, you raise pre-flop with AQ, flop comes A95r, you continuation bet and get called. There are no draws to speak of, so either Villain as AJ/AT and is drawing to two outs, or he has you buried with AK/A9/99/55. In the latter case, you'd obviously prefer not to put any more money in the pot, and in the former, a free card is unlikely to hurt you. Note that this is different from the KK hand, where a free card could be very bad for Hero. That made Sirio's decision much harder, but ultimately he determined that keeping the pot small was more important.