Although the Apostles are often referred to as the first ‘bishops’ of particular areas, there is a significant difference between Apostles and bishops.
Apostles, personally commissioned and sent by our Lord Jesus Christ, have apostolic authority wherever they went. Bishops, appointed as their successors, have apostolic authority as well, but only within their own diocese (or metropolis, which is just a large metropolitan diocese).
St. Jerome of Stridonium tells us;
Wherever there is a bishop, whether it be at Rome or at Engubium, whether it be at Constantinople or at Rhegium, whether it be at Alexandria or at Zoan, his dignity is one and his priesthood is one. Neither the command of wealth nor the lowliness of poverty makes him more a bishop or less a bishop. All alike are successors of the apostles.