The Rishonim write that the Mal'ach (angel) Micha'el is called the "great officer," based on the Gemara at the end of Menachos which says that his responsibility in the upper heavens is "to offer 'Korbanos' from the souls of the Tzadikim." Accordingly, he is like the "Kohen Gadol" among the other Mal'achim.

We also find that he is ascribed "greatness" in that he is the angel who defends the Jewish people against the accusations of the angels of the other nations, as we find in Yoma 77a.

Regarding the name "Micha'el," the Midrash (Bamidbar Rabah 2:10) says that he is called "Micha'el" because his role expresses the lofty praise of, "Mi k'Kel Yeshurun" ("Who is like the G-d of Israel").

M. Kornfeld