Kommentar zu Bereschit 10:12

העיר הגדלה THE GREAT CITY — This refers to Nineveh (not to Calah the last mentioned city nor to Resen), as it is said (Jonah 3:3) “Now Nineveh was an exceeding great city” (Yoma 10a).

ויבן את נינוה היא העיר הגדולה, most commentators believe that the words העיר הגדולה refer to the city Nineveh, seeing that this city has been described in similar terms in Jonah 3,3. I believe that actually, these words refer to רסן, seeing that our verse speaks about this city in the first place. The entire story, as we mentioned previously, has as its objective to demonstrate how ownership of even a sizable land or city is subject to rapid change. Even though Ashur established a great empire, this empire was crushed by the Babylonians who replaced them, and who, in turn were replaced in short order by the Persians and the Medes.

This refers to Nineveh. [Rashi explains this] because we do not know if “the great city” refers to Nineveh or to Kalach. [And Rashi knows it is Nineveh] because if Kalach was “the great city,” it should say, ובין כלח העיר הגדולה. Why does it say היא? (Nachalas Yaakov, citing Tosfos Yoma 10a, ד"ה איני)