For mobi files this is often due to the book using <blockquote> instead of <p> tags. Or it could possibly be due to some general page margin setting.
As I commented on the other thread, my preferred approach is to convert the book to epub, edit the book using either Tweak ePub/Sigil or the Modify ePub plugin to get it looking exactly how I want it, and then convert back to mobi. That way you can edit the .css file of the epub to your liking for all margins.
Someone may have a more direct approach for mobi files though.