I've tried to do lossy compression of epubs with some lines of bash scripts; i.e. removing the images and fonts that were not needed. Many epubs could be downsized to a third of their size, but then I found a book that needed the supplied fonts and gave up. When doing lossy compressions can not have those kind of bugs.

What I also found was that many of the images in the epubs were already unuseable and nothing like their counter parts in phsyical books.