In ויקרא כא:י the Torah tells us והכהן הגדול אשר יוצק על ראשו... מאחיו -- about the Kohen who was greater [Gadol] than his brothers. Why was he called the כהן גדול ?
He was called the High Priest because he is superior in five things -- wisdom, strength, beauty, wealth, and appearance (see Rambam, Hilchot Klei Hamikdash 5:1). As regards to beauty and appearance, he should be the fairest and most imposing among his brothers. As regards to wisdom, he should wiser than his brothers. When it says he was superior in strength, he had to be physically strong. If one takes the Torah in Bamidbar (8:11) literally, Aharon had to lift 22,000 Levites (one at a time, of course) and wave them forwards, backwards, up and down. He also had to be richer than his brothers. If he was not independently wealthy, then his fellow priests had to provide him with money.
A story is told of Pinchas that when he was appointed High Priest, he was a stone cutter and he was not wealthy. When his fellow priests came and saw him cutting stones for money, they put their gold in his basket for him to become rich.
Where does it say his fellow priests should provide him with money? The source is the same pasuk we quoted at the beginning: "And the priest who is greater through the instrumentality of his brethren." Many of these rules also apply to a king -- he should have characteristics similar to those that a כהן needs to be appointed Kohen Gadol. If he lacks these qualifications, he cannot be considered a leader (see Rambam Hilchot Melachim 2:5).