Why Is Hallel HaGadol Called The Great Hallel?

I think I just discovered why. In the very beginning of the megillah (whose masechta we will be starting tomorrow), we hear about the impressive span of the countries over which King Achashverosh rules:

א וַיְהִי, בִּימֵי אֲחַשְׁוֵרוֹשׁ: הוּא אֲחַשְׁוֵרוֹשׁ, הַמֹּלֵךְ מֵהֹדּוּ וְעַד-כּוּשׁ--שֶׁבַע וְעֶשְׂרִים וּמֵאָה, מְדִינָה.

1 Now it came to pass in the days of Ahasuerus--this is Ahasuerus who reigned, from India even unto Ethiopia, over a hundred and seven and twenty provinces--

Apparently, Hallel haGadol has a similarly impressive span:

On Rif Taanit 9a {going on Taanit 26a}

ואיזהו הלל הגדול רב יהודה אמר מהודו עד על נהרות בבל
וקיימא לן כוותיה

And what is the Great Hallel? Rav Yehuda said: From Hodu {Tehillim 136}:

א הוֹדוּ לַה כִּי-טוֹב: כִּי לְעוֹלָם חַסְדּוֹ.

1 O give thanks unto the LORD, for He is good, for His mercy endureth for ever.

until "Al Naharot Bavel" {Tehillim 137}

א עַל נַהֲרוֹת, בָּבֶל--שָׁם יָשַׁבְנוּ, גַּם-בָּכִינוּ: בְּזָכְרֵנוּ, אֶת-צִיּוֹן.

1 By the rivers of Babylon, there we sat down, yea, we wept, when we remembered Zion.

{presumably only chapter 136}
And we establish like him.

It is not meHodu veAd Kush, from India to Ethiopia but rather meHodu veAd Al Naharot Bavel, until the banks of the rivers of Babylon.

;)