[JSWolf]

Some eBooks such as those from Penguin Random House have useless fonts embedded. They have one XHTLM file that uses those fonts and that file can be deleted as well as the associated fonts. Also, I reduce the size of the cover and titlepage graphics down to 1600 lines. And in most cases, I'll losslessly compress at 80% as most of the graphics used it won't be an issue. I'll also remove any extra bits like a review page, any advertising and any associated graphics. Also, any previews go along with the graphics (if any). Also gone is about the publisher and books by the author. I'll also remove unused CSS as that can make the book perform better without the sum times hundreds of unused CSS. Also, if there are embedded fonts that I'm not deleting, I'll subset them. After all of this, I can significantly reduce the some of some ePub. I'm using the Calibre editor for these things.

As for Mobi, you cannot edit them, so don't start with Mobi. Saying that, you can do these sorts of edit on KF8 (AZW3). If ePub is your source and a Kindle supports KF8 or KFX , convert to KF8 or KFX (if it works using the KFX conversion plugin and your device supports KFX) and forget Mobi exists.