J.M.J. Venerable Bede, in his Commentary on Luke, offers the following.
The Almighty has done great things for me, and holy is His Name.
Mary looks back to the beginning of her song, where she said: My soul proclaims the greatness of the Lord. Only that soul for whom the Lord in His love does great things can proclaim His greatness with fitting praise and encourage those who share her desire and purpose, saying: Join with me in proclaiming the greatness of the Lord; let us extol His Name together.
Those who know the Lord, yet refuse to proclaim His greatness and sanctify His Name to the limit of their power, will be considered the least in the Kingdom of Heaven. His Name is called holy because in the sublimity of His unique power He surpasses every creature and is far removed from all that He has made.