You can lose a lot of money with the smaller overpair in situations like these, I tend to err on the weak side and give a preflop reraiser credit for a big pair unless I have information that tells me otherwise. I call the flop and reevaluate on the turn, if he shows weakness I raise him there or I let it go if I read him as strong. You also give yourself a chance at hitting that gutshot or one of the two nines left in the deck.