The book 1491: New Revelations of the Americas Before Columbus, claims that the corpus of texts in Nahuatl is larger than the corpus of Classical Greek texts. Here is the full quote:
”Yet Athens had a coruscating tradition of rhetoric, lyric drama, and philosophy. So did Tenochtitlan and the other cities in the Triple Alliance. In fact, the corpus of writings in classical Nahuatl, the language of the alliance, is even larger than the corpus of texts in classical Greek.”
When I read this I was immediately sceptical. A citation is provided, specifically to someone named Frances Karttunen, but no specific work is cited. How accurate are these claims?
There is, in fact, a dizzying amount of Nahuatl texts: even as somebody who has done a deep dive into Mesoamerican history and culture and regularly makes detailed posts about it; and tried to track down as many primary sources as I can, there's so many that it's hard to to keep track.
However, I'll refer you to an existing answer to this very question by /u/400-Rabbits here, though this admittedly talks more about the context of these Nahuatl documents and gives some examples, as the core question is sort of hard to definitively answer.
TL;DR That claim is kinda misleading. Also: What a weird thing to compare. Like, I get it. I get that the author has an agenda—that he or she is trying to overcompensate for thousands of years of Eurocentrism, but these kinds of comparisons—and their weird implications—are not a constructive way to go about demolishing stereotypes. Well... unless your audience is full of mouth-breathers who believe: Moar Corpus = Moar Valid.
Our most informative “preconquest” documents were mainly redone under Spanish auspices in the 1540s and later. You can read a list here. It's mostly annals, king lists, and divination/calendrical topics.
It's during this time that Spanish ecclesiastics were reducing spoken Nahuatl to the Roman alphabet and beginning to teach some of their Nahua student-aides how to write in that fashion. Thus, from the 1540s onward we begin to see more and more civil documents produced in alphabetic writing. By 1570 even the smallest city had a clerk or a notary attached to a church who kept records such as land purchases, the emoluments of altepetl officials, taxes, tributary lists, last wills and testaments. As James Lockhart puts it:
Postconquest pictographic documents have received more than their proper share of attention, thanks in part to collectors who mistakenly thought them preconquest and thus filled the European archives with them.
As a result of their easy availability, many have been published, giving an inflated impression of their weight in the overall scheme of things. Actually, their bulk is infintesimal compared with the mass of Nahua-produced documentation existing in the Mexican national archives, where the alphabetic Nahuatl corpus, already many times as large as the pictographic, is growing daily.
To be clear, this alphabetic corpus consists of mostly of “mundane” documents. The earliest known corpus of alphabetic Nahuatl are the Cuernavaca-region census records of the late 1530s. Other common examples of the corpus are court records, petitions and complaints to the Spanish authorities, cabildo meeting minutes, and personal correspondence in Nahuatl. As S.L. Cline puts it in her book The Testaments of Culhuacan:
Many local-level Nahuatl documents from the colonial era survive, a large number of them recording transactions by ordinary people. The majority of these documents are European legal instruments of a type introduced after the conquest, but readily adopted by the Nahuas such as wills.
So is your quote technically correct? Sure, but it's misleading. The Nahuatl corpus is only larger because more of it has been preserved (it's from the 1500s CE for crying out loud, not the 500s BCE!) and most of it is civil documentation. For more on this topic I highly recommend:
James Lockhart's The Nahuas After the Conquest
S. L. Cline's Colonial Culhuacan 1580-1600
and her other work The Testaments of Culhuacan.