Thumbnail for cover image has white borders

[Mookiemon]

I'm sideloading an epub to my Kobo Touch for testing before I upload it to sell in the Kobo store. My aspect ratio is 4:3 (800 pixels high by 600 pixels wide) and when I open the ebook, the full page cover looks fine. However, when viewed as a thumbnail on the home screen on the Kobo, it has white borders at the top and bottom of the cover image. All the other covers (all bought from Kobo) look fine. My aspect ration is being preserved (the image is not distorted) but the thumbnails make it look like the cover image needs more height. I've tried deleting all the files pertaining to this ebook from my Kobo, ejecting the Kobo and re-dragging the epub over, thinking perhaps the thumbnail is an earlier version, but the problem persists.
Here is the code I'm using for the cover:

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 600 800" width="100%">
      <image height="800" width="600" xlink:href="../Images/cover.jpeg"></image>

Thanks in advance if anyone has any suggestions!