This might be more of an r/linguistics thread question than one for historians - vocabulary measurement is incredibly complex and hard to pin down, and your numbers (20K-35K) are estimates with certain ideological assumptions built in. For example, claiming that a "normal" person in 1600 had a quantitatively different vocabulary than someone in 1500 suggests some pretty profound assumptions about language change and human cognitive capacity that we really don't have any justifiable expectation of. That qualification can be extended as easily to the difference between ca. 2000 and ca. 2000 BCE.

The problem also hits the heart of the idea that some words are wordier than others - there is an implicit reading of "well-educated" in your question, which suggests some divergence between what qualifies as words - someone with a terminal degree might use many more words than someone on the street, but aside from outliers and word-collectors, most humans have a similar baseline - although whether some view "bae" as a word compared to "solipsistic" is an ideological question.

Difficulties among "words" themselves: do "individual" words count, like "nation" and "nationality" or do you count each word family only once? How do these derivational vs. morphological differences count ("dog" and "dogs")? What are the guidelines? There's frankly no measure of even a rough generalization of this historically in the record, not even currently.

We might be able to ask you, are you asking if there is some cognitive difference between modern and early-modern humans? Or is this a question of access to media and literacy materials/technology? And as regards your question, a more fruitful focus might be the writers themselves - did they conceive of their audiences having more or less limited literacy? Because literacy, as in New Literacy, is a modern invention, of literacies as multiple and not determined by education and genetics and class.