"If you bet so little that it's +EV for your opponent to call, then it's -EV for you... Poker is a zero sum game..."

that's not exactly true. If the pot is 14 million dollars and you have 1 dollar left, even if a flush scare card hits and threatens your set, if your opponent puts you all-in for your dollar on the river, it is +EV for him to put you all-in with a flush, and +EV for you to call with your set, because odds are greater than 1 in 14 million that your hand is actually good.