While the ideas surrounding QDM lean towards the harvesting of mature animals, you should always set your goals according to what you want to harvest. If you are happy with a 120 class animal, then QDM to you is pasing on smaller yearling bucks with the hope of them growing into the kind of animal that you want to harvest. While it may be true that other hunters may harvest animals that you have let walk, if you kill them you know for certain that they will not live. If you have seen larger deer in your area, it's proof that they don't all get killed, and the one you let walk may be the one that you see at a later date.