How did the boundaries of the Byzantine Empire change?

The boundaries of the Byzantine Empire were changing throughout its reign. The boundaries consistently expanded and shrank depending on the circumstances of the empire. For example, at its greatest extent under the conquest of Justinian I, the Byzantine Empire owned everything from Carthage to Constantinople. You could even say the empire "Owned" the Mediterranean Sea. Near the end of the empire, when it was consistently being bombarded with sieges from the Ottoman Turks, the Byzantine Empire only consisted of one city, and that was the capitol Constantinople.