Actually in many cases the larger the tree the less time it takes. For example I find it much easier to wire a larger tree, and in some species it is easier to keep them maintained. It really depends on the tree and the species (as well as point in development), but size is not directly related to work required. Try to maintain a shohin broom-style elm for example... hoo boy! Compared to a large California juniper that just gets pruned once per year and needs only light finished wiring on occasion.