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I'm not sure our ebook team can get the file size much lower than that. Amazon's format for the mobi files requires that it contains the ePub format as well, for compatibility, so that makes them larger.
For mobi files larger than 50mb, you'll have to use an alternate method for getting the mobi on your Kindle, such as downloading it directly from your oreilly.com account to your Kindle, or plugging the Kindle into your computer and copying the mobi file to the Documents folder on the Kindle.
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