The problem is definitely with the previewer. I had a previous version of the previewer, and all my mobis were fine. Then I updated it, and now all my covers (Kindle & Kindle DX) are cropped.
Also, I emailed my latest mobi to my actual Kindle. It was fine on the Kindle, cropped in the previewer.
I'm now trying to find out where I can download an older Previewer.