There are two possible explanations for this: (1) the number of documents withheld in cross-validation decreases with the number of documents, and stability depends more on the number of documents that differ between cross-validations than on the fraction of documents that differ, or (2) a fixed number of schemas can better capture the information contained in a smaller set of documents because new documents add more novel and unique narratives, thereby making the content of the narratives more difficult to capture in a finite set.
provided with more and more documents, the germinators do not converge to some finite set of schemas, but instead are presented with an increasingly difficult problem to solve.