The .mobi file is larger than the rest because it contains three different versions of the file (!).The two ePub files and the .mobi file have been generated byVellum, an elegant piece of ebook authoring software. The .azk file was made using Amazon's Kindle Previewer software so the file can be loaded onto the iPad Kindle reader. The .azw file is AZW3 (or K8) and generated by Calibre, an ebook library and conversion program. If you have any problems or need a different kind of file please contact me.