[kovidgoyal]

This is because your book specifies extremely tiny font sizes (50% of normal), while the jacket uses "normal" font sizes. Currently, the jacket insertion code does not try to alter the font sizes int he jacket to match the font sizes int he rest of the book, it is possible to have it do that, but doing that would mean a loss of control for people trying to customize the look and feel of the jacket.

In your case, you might be able to work around the problem by converting epub to epub without a jacket to normalize the font sizes and then converting again with a jacket (after deleting original_epub so that the newly produced epub is used).

Or just edit the source file to remove the silly repeated font-size: 70% declarations from the stylesheet.