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Old 08-17-2011, 06:00 PM   #1
perdman
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18MB EPUB converts to 40MB Mobi! What can I do to reduce Mobi file size?

Hi everybody, I'm new here on this forum. I design books in Adobe InDesign for print and ebook publishing and have been using Calibre to convert EPUBs to the Mobi format. Usually this goes quite well, but today I converted a very large EPUB, weighing in at 18MB, to MOBI, and the resulting MOBI was 40MB! I even did the conversion several times hoping it would make some difference, but to no avail.

My standard workflow involves exporting the EPUB from InDesign CS5.5 with export tagging enabled for all styles, using a TOC Style to generate the navigation, with no embedded fonts, and no generated CSS. I unzip the EPUB contents and insert my own handwritten CSS file and go through the HTML to double check it. When I'm all done I zip it all back up and run it through a validator, then I use calibre to make a mobi of it. Both the EPUB and Mobi are submitted for final publication (usually along with the final print PDF and sometimes online/interactive PDF as well).

This particular file has 184 JPG/PNG images totaling 19.2 MB uncompressed; the entire uncompressed EPUB folder is 21.7 MB. Whatever is happening during the mobi conversion is adding artificial bulk to the file; I've often seen file size increases when converting to mobi, but never more than doubling of file size. Jumping from 18MB EPUB to 40MB MOBI is a bit ridiculous. Anybody have any idea what I might be able to do to reduce the file size?
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Old 08-17-2011, 06:22 PM   #2
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Also, I just found and tried the MobiPocketUnpacker script from here:
http://www.mobileread.com/forums/sho...36&postcount=5

but got errors when when attempting to run it using the drag/drop interface described. I'm using Mac OS 10.6.8. Unless there is another tool for unpacking the MOBI file, it might not be possible to open it after conversion.

The Mobi seems to open and display properly in Calibre, apart from taking a while to load.

I should mention this book is a bit of a monster; in print, it's an 7x10 800+ page legal reference with several appendices made up of placed PDF docs, which accounts for most of those images. Unfortunately InDesign rasterizes all the placed PDF content during EPUB export and saves it out as PNG or JPG images.
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Old 08-17-2011, 06:48 PM   #3
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You could try using kindlegen to convert the epub, it might or might not produce a smaller output.
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Old 08-17-2011, 11:10 PM   #4
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It gets weirder. I found a free utility called PNGenie that successfully reduced the collective size of my 206 PNG images from 19MB to 10MB. My EPUB is half the size it was before, yet when I convert the new EPUB to a MOBI in Calibre, IT IS STILL 40MB. That's Just Wrong. Cutting 10 MB from the images should have cut at least 10 MB from the MOBI file size. something is definitely wrong here.

Last edited by perdman; 08-18-2011 at 12:32 AM.
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Old 08-17-2011, 11:17 PM   #5
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All images in MOBI files are stored as JPEGs. For certain classes of image, jpegs are much larger than PNGs. Various MOBI renderers have various problems trying to render PNG images.
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Old 08-17-2011, 11:18 PM   #6
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It could also be that the Mobipocket is not being compressed. The problem is that the HTML code is bloated in Mobipocket due to it's inability to use CSS. So every line has to specify what the CSS would specify once.

Last edited by JSWolf; 08-17-2011 at 11:21 PM.
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Old 08-18-2011, 12:24 AM   #7
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Thank you for your responsses! So if I understand correctly, the file size is bloating up to 40MB because (1) my CSS has to get merged into the HTML and (2) all the tiny PNGs get blown up into JPGs? Furthermore, the file size reduction I did on my PNGs would have no effect, because MOBI does not support PNG? I guess this makes sense.

Makes me hate the MOBI format all the more. I just wish Kindle would get with the program and work with EPUB a bit more nicely! Bloating a 10MB file all the way up to 40MB is really killing this.

I also tried converting the EPUB using KindleGen. It was my first time using it, as I generally prefer a GUI. The result is 25 MB, which is certainly an improvement, but it did a poorer job preserving formatting than Calibre usually does. I will continue to use Calibre for most books, but it's unfortunate that it has made a mess of this one. It's also unfortunate that the Mobi format is so much harder to work with than EPUB; I'm constantly unzipping, tweaking, and rezipping my EPUBs until I get them just right; wish I could more easily do that with my Mobis, too.

I'll send both the 40 and 25 MB files to the publisher and let them choose which one to use. Thanks for your advice!
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Old 08-27-2011, 03:40 PM   #8
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Have you tried Amazon's Kindle Plugin for Adobe InDesign® v0.93 to create MOBIs directly from InDesign?
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