[GeoffR]

If you use Calibre to send the covers there is a 'Send balck and white covers' option in the driver that will convert the cover to greyscale before sending it to the device. I think this makes the filesizes about 10% smaller. (The cover images for books synced from the Kobo cloud are greyscale already.)

Edit: Also be aware that Calibre automatically replaces the cover image inside the ePub with the one you have set as cover in Calibre as a part of updating the metadata when it sends the book to the device. Often these are the same image and so there is no effect, but it means that if you edit the ePub to reduce the size of a large cover image inside the book, it will just be replaced with the original one that was in Calibre when the book is actually sent to the device.

Edit2: There is also a problem with the way Calibre treats books that have PNG covers, it will convert the PNG to JPG when it adds the cover to Calibre, then convert JPG back to PNG to replace the cover in the book when sending the book to the device. This can sometimes result in a large increase in the filesize of the cover inside the book because of the JPG to PNG conversion.