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?
On
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.
On
.epub files are compressed (in fact, they are just zip files).
.mobi files are not compressed. If you zip a mobi file, you may get a smaller file than the epub.
Incidentally, this makes text searching much faster on mobi files than on epub.
On
That depends on the format of the mobi that you have. As you must be already aware, an epub file can be converted into any ebook format that you choose - you can consider the epub format as the base for any other format.
I am guessing that the mobi file that you have has the original epub embedded inside it. This is to assist editing tools (as direct editing of mobi files is cumbersome). Also, some mobi files contain several versions of the mobi(mobi-7 and KF8) to maintain backward compatibility with readers that do not support the latest format.
You can find more information about the file formats here
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.