Of course, one could make the argument that the Enlarge spell should increase one's size category with all of the benefits and penalties that go with it as described in the MM. After all, since the rules for changing size categories is set in a core rulebook, why should a spell which effectively changes your size category be a special exception? Obviously, this would make the Enlarge spell more powerfull than it is now and would warrant a level increase, but I don't see anything wrong with doing this or perhaps making a separate higher level enlargement spell.