I have an ornamental lilac tree that is in a round shape. I reshape it every year but it's round shape gains increasing volume. I am afraid to reshape it aggresively down to a smaller shape because the leaves grow only on the outer area and do not grow on the inner branches. If I cut too much, the tree will have no leaves. It's location is impeding a traffic area beside a driveway and is becoming a problem. The only thing I know about pruning the tree is that I am supposed to prune the blossoms after flowering.