When dealing with the HathiTrust public domain volumes, the makeup of the collection changes drastically. This is because US copyright law makes it difficult to ascertain rights post-1923, resulting in representation biases. There are proportionally less public domain works approaching the present day, but the drop-off is much quicker in some categories than others. For a valid understanding of the collection, one shouldn't compare pre-1923 and 1923- subsets of the public domain collection without additional context.
Figure: Distribution of Categories by Year. 1923 is marked with a vertical line.
Table: Library of Congress class distributions for Public Domain data, in % of Collection