מגדל [מַגְדִּיל֮] יְשׁוּע֪וֹת מַ֫לְכּ֥וֹ וְעֹ֤שֶׂה חֶ֨סֶד ׀ לִמְשִׁיח֗וֹ לְדָוִ֥ד וּלְזַרְע֗וֹ עַד־עוֹלָֽם׃
Great salvation giveth He to His king; and showeth mercy to His anointed, To David and to his seed, for evermore.
Radak on Psalms
Magdil ["{God} magnifies"] salvations. Magdil, and in the Book of (II) Samuel (22:51), [it is written as] migdol ["a tower"], and it is a descriptor--but the idea is one [and the same].
Metzudat Zion on Psalms
Magdil ['magnifying']]. From the language of gadol ['magnanimous' or 'great'].
Ibn Ezra on Psalms
Magnifying: "And to Your name shall I sing," (Psalm 18:50) for it "magnifies" (Psalm 18:51).