Originally Posted by RG1
During cover image generation the margins you have set in preferences will be added. I have used a class for body to get around this. You can prevent kobos own margins in generated images if you set the preference for margins to zero before adding a book. This will work for books stored in main memory only. (The images for books stored on sd card are generated on-the-fly and are not persistent.)
This explains why the covers are squeezed. It isn't that the publisher is adding the margin, but the kobo is assuming that the same margin that applies to the page applies to the cover (this seems silly, but maybe that is just me).
Unfortunately, with recent firmwares, the cover generation is done on the fly, rather than all at once when books are added. This means that I will either need to either find a way for each publisher's ebooks to set the margins for the cover to 0, so I can adjust the margins as desired when reading and this won't affect the generation of new covers. Or find a way, again for each publisher, to hard-code the margins of each book, except the cover, so I can keep the kobo margin settings at 0 (this may be simpler, but mean's I can't change the margins when reading, which is a pain).