I recently worked on a rather large EPUB project. After downsizing the EPUB’s images with the PNGenie utility I was able to get the EPUB’s size reduced from 18MB down to 10.5 MB. However, what disturbed me most was that when I converted the EPUB to a Mobi file the size was still 40MB!
But first, let’s back up. What’s a Mobi, how do we get to Mobi from EPUB, and why would be want to, anyway?
EPUBs are the most popular eBook format on the market, being supported on almost every major eBook reader device. The one notable exception is the Kindle, from Amazon.com, which only supports its own proprietary format as well as a few others, including…you guessed it…Mobi. The MobiPocket Reader was the precursor to the Kindle reader. It’s a very old file format, and as such it does not support many of the newer technologies that the EPUB format supports—such as PNG graphics and CSS.
I learned recently that it is partly because of these limitations of the Mobi/Kindle file format that you will usually see an increase in file size when converting your EPUB. The converter must write all of your CSS into every instance of each respective HTML tag. In my case, this isn’t as bad as it usually would be coming from InDesign, as I use custom style mapping and write my own CSS to be as lean as possible, but it still can be a problem. It would be much worse if you let InDesign create the CSS automatically for you. In addition, JPGs are often larger in file size than PNGs. And then I’ve also read that some Mobi converters do not optimize the compression of the data as well as others, leading to larger file sizes than necessary.
I’ve worked with two free tools that convert EPUBs to Mobis. My preferred method is Calibre, which is easy to use and does a reasonably good job of preserving my formatting. In this case, it created a 40MB Mobi file from my EPUB, even after I used PNGenie to reduce the size of the images. I was baffled until I learned what I described above (all PNGs are converted to JPGs in Mobis, so reducing their size would have no effect on the size of a Mobi). I then tried a utility I had not tried before called KindleGen, a command line converter available directly from Amazon. I usually steer clear of command-line tools, but being rather desperate to get the file size down I gave it a try; the resulting file size was 25MB, but the file didn’t look as nice as the file from Calibre.
I should mention that Amazon’s site also has a beta plugin for InDesign that supposedly exports directly to Kindle. I was skeptical at first, but after reading some pretty good reviews by some rather well known people here, I think I might give it a try. Still, everything is working well now, and the EPUB=>Mobi conversion in Calibre works well enough for me now…I will probably wait for a slow moment to install the plugin. Stay tuned; I’ll write a review when I get a chance to try it out!
Note: some information in this post I learned from the mobileread discussion forum. My thanks to Ron, Kovidgoyal (creator of Calibre), and JSWolf for your helpful suggestions.