Magnus, meaning "Great" in Latin, was used as cognomen of Gnaeus Pompeius Magnus in the first century BC. The best-known use of the name during the Roman Empire is for the fourth-century Western Roman Emperor Magnus Maximus. The name gained wider popularity in the Middle Ages among various European peoples and their royal houses, being introduced to them upon being converted to the Latin-speaking Catholic Christianity. This was especially the case with Scandinavian royalty and nobility.
https://en.wikipedia.org/wiki/Magnus