[phossler]

What I can't figure out is why the file size doubles in size when I convert the epub to Kindle format.

http://velluminous.com/handcraftedebooks/archives/78

If you’ve worked with many ePubs and converted them with Kindlegen while paying attention to file sizes, you might well have noticed that the Kindle files are much larger than the original ePubs — typically around twice as large. It turns out that Kindlegen stashes a copy of the original ePub sources inside its output file, which explains the doubling of size. Unfortunately, this can lead to super-sized Kindle files, particularly in heavily-illustrated works, since images get duplicated along with everything else.

Paul