From InDesign to EPUBs to Mobi (Kindle)—and why Mobis are BLOATED

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, 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.

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3 Responses to From InDesign to EPUBs to Mobi (Kindle)—and why Mobis are BLOATED

  1. Amlaith December 7, 2011 at 7:46 am #

    Thank your for this very informational post. At first, I thought I made a mistake. I am new to all of this, never worked much with Indesign CS 5.5 before. So I tried the latest Kindle Plugin-Version and exported my “Testbook” into .mobi.

    After that I scratched my head – I only had included our Cover, a TOC (which was also a bit problematic) and a first glimpse of the first chapter of the book with two pages text. And still my .mobi was bigger in file size than another book that had a cover (around the same file size) and hundreds of pages. (my version was 900 kb; the other one around 500 kb with cover, chapters and hundreds of pages)

    There so many things to think about right now. Like: I cannot rely on any tests I made with Kindle and Calibre. Why? Because it seems that if you upload a file to Amazon and then download the preview version for the Kindle – it looks different than the USB-direct-cable-version.

    I still hope that I can reduce the file size and still rescue some of my formating.

    Thank you. Greetings from Germany

  2. Robert Stoothoff January 25, 2013 at 12:59 pm #

    In my (limited) experience, Calibre works very well in converting .epub to .mobi – better, indeed, than Kindlegen and the InDesign Kindle plug-in (which is better now than it was, but still not so good as Calibre). The problem is, that Amazon finds some Calibre-created .mobi files unacceptable, for reasons which it will not reveal. But if you want to submit a book file to KDP, of course, there’s no need to upload a .mobi file: you can upload an .epub, and let them do the conversion. No need to mess about with the niceties of .mobi.

    • Paul Erdman January 25, 2013 at 1:42 pm #

      I wrote this article quite some time ago. Since then I’ve been working on catalog production and other projects and haven’t done an ebook for a while. I’m in the process of generating a couple EPUBs now using InDesign CS6 and revisiting the Mobi/Kindle situation as well, and will be researching and writing an updated article soon. Thanks for your comments. At the time I wrote the original, I’m pretty certain it was not possible to upload an epub and let amazon do the conversion for you, as stated in your post, for example. That would be an exciting new development, and I’m excited to look into it.

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