Plus, by retaining the ruling class at a high standard, they would be set socially apart from those whose task it is to rule, which is indeed proper. A good ruler cannot establish total control if the commoners regard the ruler as a peer. Rather, the ruling class must be revered, and providing them with a lavish lifestyle is the best way to do that. And nobody can dispute that it is only fair to reward those whose task it is to make the decisions that the commoners lack the talent to make for themselves.
Thus, it is only fair if all commoners live at a meager level, otherwise people will be envious of each other. And it is only fair that our rulers live better than the rabble they have to care for.