Why is Lord Vishnu called Mahavishnu?

Why is Lord Vishnu called Mahavishnu? Maha Vishnu is usually referred to his multiversal form when he breathes and creates infinite universes in vaikunta : As a garbhavishnu expansion he enters each universe he created as Brahma Life of Brahma is equal to one inhalation of Vishnu's breath . Universes gets created and destroyed by Vishnu's exhaling and inhaling of the universes . Mahabharata describes it that existence and non existence are a part of Vishnu consciousness : “He is Iswara (Supreme Lord). It is from me, that the entire mobile and immobile universe springs. It is from me, O Narada, that the indestructible and destructible, the existent and the non-existent, flow. They that are devoted to me enter into me and become emancipate. I am known as Purusha.” Krishna /Rama are no different than Mahavishnu