Cover Art + Calibre + Kindle Paperwhite 3 - Problems

Hello All,

I am using Calibre 2.31.0, and for the AZW3 that I have converted from EPUB, when I transfer the AZW3 to the latest Kindle Paperwhite 3 via "Send to Device", many of the books have different sized thumbnails in "Cover Flow". Some are big, some are small, but they aren't uniform, and it looks very odd.

I never had this problem with the Kindle Paperwhite 2.

Is anyone else seeing this, any solution for this?

Thanks!