If he calls turn we have slightly less then pot left. Also since the board pairs it's getting even more unlikely he has a draw and more likely he has something with showdown value. I doubt he'll call off his stack with less then a Q here or try anything fancy if we check(like turn a hand with showdown value into a bluff). A smaller bet (like 66) leaves us with an awkward stack and the problem is it looks to much like we want to get value