Regular Kohen on the left, Kohen HaGadol on the right (front and back)
-Chabad.org
לקדשו לכהנו לי TO SANCTIFY HIM, TO APPOINT HIM AS PRIEST TO ME — to sanctify him, i.e. to install him into the priesthood by means of the garments here specified, so that he may become priest unto Me. The expression of כהונו chahano denotes service
(ח) ולהגדיל הבית עוד - הגדיל מעלת עובדיו ונבדלו 'הכהנים והלוים' וציוה להלביש ה'כוהנים' בגדים נאים ומלבושים יפים וטובים "בגדי קודש לכבוד ולתפארת" ושלא ישמש ב'עבודה' 'בעל מום' ולא בעל מום לבד אלא הכיעורים גם כן 'פוסלים' ב'כהנים' - כמו שהתבארה בתלמוד זאת ה'מצוה' - מפני שההמון לא יגדל אדם אצלם בצורתו האמיתית אלא בשלמות איבריו ויפי בגדיו; והמכוון - שתהיה לבית גדולה ותפארת אצל הכל.
In order to raise the estimation of the Temple, those who ministered therein received great honor: and the priests and Levites were therefore distinguished from the rest. It was commanded that the priests should be clothed properly with beautiful and good garments, "holy garments for glory and for beauty" (Exod. 28:2). A priest that had a blemish was not allowed to officiate; and not only those that had a blemish were excluded from the service, but also--according to the Talmudic interpretation of this precept--those that had an abnormal appearance; for the multitude does not estimate man by his true form but by the perfection of his bodily limbs and the beauty of his garments, and the temple was to be held in great reverence by all.
Avivah Gottlieb Zornberg, The Particulars of Rapture: Reflections on Exodus, New York: Doubleday, 2001, p. 369.
The High Priest's vestments invest him in anxiety, no less than in glory. Ultimately, it is not only the diadem that is to be "Holy to God," but its wearer. If the dissonance between vestment and wearer is palpable, if there is not "that within which passes show," the trappings become hollow.
The Lubavitcher Rebbe
And the Breastplate shall not budge from the Efod (apron) (28:28)
The Efod (Apron) was worn in back and below the waist; the Choshen (Breastplate,) on the front and upper part of the wearer. Thus, the deeper significance of the commandment "And the Breastplate shall not budge from the Efod"... is that there must be no "gap" between the upper and lower aspects of life, ... True, the human being consists of both the sensitive heart and the functional foot; true, life is composed of sublimely spiritual moments as well as the daily tending to one's material/physical needs. But the "Efod" must be securely bound to the "Choshen". The upper must permeate the lower, and the external must never lose sight of its inner essence and purpose.