I have now confirmed that the two books which display on the Fire HD with very wide right margins are KF8 books. My DLed purchased mobi7 books seem to render the right margin correctly. so this issue seems to be another case of Amazon screwing up the rendering of KF8 books on certain devices. On the Kindle Touch, KF8 books render with a large line spacing. On the Fire HD they render with a large right margin.
Has anyone noticed this on the original Fire? Here is link to the sample book on Amazon.com which is KF8 and renders a wide right margin if you want to test it:
The Story of Britain: From the Romans to the Present: A Narrative History [Kindle Edition]