Magdil and Migdal A Growing Tower

The final Harachaman of bentching concludes, like so many other tefillos, with a mention of the era of Mashiach. In this case, it uses the pasuk of magdil/migdal yeshuos, one version of which is found in Tehillim and one in Shmuel Beis. The Yalkut explains that the yeshua for Klal Yisrael will not come suddenly, but will grow gradually over time, magdil. Then Mashiach will be established as strongly and powerfully as a tower, migdal. On weekdays, when Hashem so-to-speak works on the yeshua, we say magdil, but on Shabbos and Yom Tov, which are a reflection of the times of Mashiach, we say migdal.

Adapted from World of Prayer by Rabbi Elie Munk