I noticed today that when I change preview devices from Kindle to Kindle Fire, Kindle Previewer shows a very strange PDF-style view of my book. Watch this video and you'll see.
Notice that I start by opening a book and viewing it with Kindle Fire. Then I switch to Kindle Touch (looks fine), to Kindle (looks fine), back to Kindle Touch (looks fine) and to Fire (looks fine). But if I go back to Kindle (still looks fine) and then jump to Fire, it shrinks the ebook and displays it at a tiny size that does not respond to font-size changes.
The solution is to open the book again. Hopefully Amazon will get this fixed soon.