[GeoffR]

Book covers come from the publisher in all different shapes and sizes, when Kobo displays them it changes the size, but leaves the shape (aspect ratio) the same. This means that covers which were originally wider than others will appear shorter.

I think the only alternatives would be either to change the aspect ratio which distorts the cover image, or to crop the cover which if done automatically will often cut something important out of the cover.

If you want to go to the bother you can stretch or crop the cover yourself to make it a similar aspect ratio to the others, and then replace the covers on the device. Calibre together with the KoboUtilities plugin is probably the easiest way to do this.

If you have a copy of the book in Calibre (or a dummy book with the same title and author metadata) then just change the cover in Calibre to your taste, using an image editing program (I use gimp). Then use the KoboUtilitues plugin to send the covers for the book to your device. After safely ejecting the device you might need to turn it off and back on again for the new covers to show up straight away.

Edit: You can also use this method to make changes in other metadata such as title, author, synopsis, publication date, etc. It is amazing how many mistakes there are in a book's metadata, I even had one where the publisher misspelled their own name (HapperCollins).