A Mega-Diocese is a diocese so enormous that a bishop cannot oversee it. Remember “bishop” in Greek is επίσκοπος (episcopos) which means “overseer.” Epi means “over” as in the word epidermis. Skopos means “see” as in the English words scope and telescope.
A shepherd should know his flock. How can a single shepherd guide his sheep without losing many of them if he has a massive flock? Wolves and predators will certainly pick off all those on the periphery and those that stray because they cannot hear their shepherd's voice calling them. Perhaps there would be less scandal in the Roman Catholic Church if the administrative regions were not so immense.
I like the idea of the Byzantine Church structure, that if more than 200 families are in a parish, a new parish is formed.
Currently here are the number of baptized in the top 4 USA archdioceses:
1 Los Angeles 4,174,304
2 New York 2,521,087
3 Chicago 2,442,000
4 Boston 2,077,487
Italy has dioceses. 116,350 sq mi and population of 60,483,973 people
USA has dioceses. 3,796,742 sq mi and population of 325,719,178 people
These numbers are for the Roman Catholic Church. I don't know the numbers for Byzantine or Eastern Catholics but it is much, much smaller. It is clear that there is no possible way for the bishop structure to actually function in the current design. Vocations would be less an issue if there was more access to the leadership. It would help keep people from being disenfranchised.