It is not as simple as file size. Other factors related to the book's construction and the
individual component files that define the display features. Because the different formats handle these factors differently, it does vary with format. It also varies by the
program used to create the file, and the options used, even within the same format.
For example; I also encountered the "File Too Large" message with an epub file made
from HTML using Calibre. This ebook had a great number of illustrations and did not
provide a separate directory for them.
But if I converted the HTML to pdb using Calibre first, then made an epub from the
pdb, no error message in the JBL. The new ebook was structured with a directory for
the images. Everything appears normally in the new ebook, including all the illustrations.