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When I buy ebooks I download all of the available formats. I've noticed that the file sizes for the various formats can be markedly different and epub is typically much smaller.
For example:
Or:
I've flipped through them and tried to confirm that the contents are the same and they seem to be (i.e. no large images missing). Can anyone explain why epub is so much smaller than the other two?
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The mobi file is so much larger because it actually contains several files within: versions for each Kindle format (MOBI7, KF8), as well as an entire copy of the source EPUB file used to create it. These multiple versions are created when an EPUB file is converted into Amazon's proprietary mobi format.
Generally the epub will be about 10 times smaller. With a PDF, not only is the text stored, but so is the location and orientation of each character. In the case of an OCR'd document, each page is stored partly as an image. On the other hand, EPUB is just HTML.
Most ebooks are mostly text and text compresses very well so EPUB is a more efficient format. If an ebook contains a large number of images then a PDF file might be smaller but it won't be as easy to read, especially on a small screen device such as a phone.
Different formats are suitable for different uses. It's a matter of using what's most appropriate for your needs. MOBI is best for Amazon platforms, EPUB is a general use format, and PDF is best for print. Keep all three in mind when you find yourself downloading an ebook.
The mobi versions can be larger because they include the legacy mobi format, the new KF8 format and a copy of the original epub, this is assuming the mobi file was generated with the latest version of kindlegen.
For the PDF's I'm guessing (and that's all it is here) that embedded fonts may be the cause of a larger file size, another thing that comes into play here is image optimisation. Depending on the image optimisation settings used when the PDF was created will largely affect the final file size.
Epub's are basically just a bunch HTML, CSS and image files with a few XML files for defining the books metadata, chapter order and table of contents navigation. The epub file is really just a zip file with a .epub extension and since it doesn't have 3 copies of the same book like the Kindle version does it will always be much smaller.
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Because the epubs are similar to a website. An epub book is made from XHTML & CSS2 & some features like CSS3, then the software that reads epub interpret that file and make a visual representation from that code.
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