title_body: Is there a good size comparison of which of different e-book formats is smaller?
upvoted_answer: This is something I find interesting. I'm going to post an answer with some findings. As I discover more, I'll expand on my answer.
  
  Old, But Still Useful Comparisons
  
  This document is fairly old (warning, link is a zip file containing a pdf) and the information may not accurately reflect how things are today. It also does not contain many of the popular formats of today (there is none of the epub formats listed and mobipocket (prc) I believe as a predecessor to .mobi). I'll list the details for historical purposes though.
  
  Excerpt from the report (Copyright 2002 by ePMA -- ePublishing Marketing Associates -- www.epmaasoc.com):
  
  
    To make this comparison, we took the Project Gutenberg plain text file
    of the book The Metal Monster by Abraham Merritt, stripped it of the
    PG legal Disclaimer and such. Next we removed the linefeed/carriage
    returns from the end of each line, so paragraphs would flow for
    different formatting, and then added basic markup with HTML, to
    provide paragraph breaks and chapter headings. Except where noted,
    this HTML file was then used to create the eBook in other formats. no
    cover or other images were included.
  
  
  NOTE: This was done for a non-illustrated volume. There was talk of doing another test in the future with an illustrated volume, but I was not able to find it.
  
  Here were the findings:
  
  Format&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Kbytes% Txt
  Rocket (fb)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;248K&nbsp;&nbsp;&nbsp;52%&nbsp;&nbsp;
  Mobipocket (prc)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;285K&nbsp;&nbsp;&nbsp;60%&nbsp;&nbsp;MS Ebook (lit)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;292K&nbsp;&nbsp;&nbsp;61%&nbsp;&nbsp;Plain text (txt)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;477K&nbsp;&nbsp;&nbsp;100%Web Document (HTML)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;487K&nbsp;&nbsp;&nbsp;102%Embiid (ubk)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;489K&nbsp;&nbsp;&nbsp;103%MS Word 2002 (doc)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;620K&nbsp;&nbsp;&nbsp;130%hiebook (kml)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;654K&nbsp;&nbsp;&nbsp;137%Adobe Acrobat (pdf)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;691K&nbsp;&nbsp;&nbsp;145%MS Wordpad (doc)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;952K&nbsp;&nbsp;200%eBook Pack Express (exe)2860K600%
  
  
  
  UPDATE
  
  This is a comparison of various ebooks I own in multiple formats. I did not create or convert the formats personally and I am unsure of the exact versions (e.g. epub2 vs epub3). I still thought this might be useful information since all books were generated by the same company (manning). These are all programming books, and as such, they have some images in them and embedded markup language. This should provide a decent comparison vs. something that is only text or mostly images.
  
  The following list of books are compared in epub, mobi and pdf (sizes are in KiloBytes, smallest bolded).
  
  
  Brownfield Application Development in .NET
  
  epub - 3,447K
  mobi - 4,115K
  pdf - 8,573K
  
  C# in Depth, Second Edition
  
  epub - 5,429K
  mobi - 6,330K
  pdf - 8,573K
  
  HTML5 for .NET Developers (results skewed by this book, see below)
  
  epub - 24,533K
  mobi - 13,945K
  pdf - 13,177K
  
  PowerShell and WMI
  
  epub - 8,797K
  mobi - 11,081K
  pdf - 14,536K
  
  Real-World Functional Programming With examples in F# and C#
  
  epub - 8,483K
  mobi - 9,446K
  pdf - 14,536K
  
  The Art of Unit Testing with Examples in .Net
  
  epub - 4,280K
  mobi - 4,754K
  pdf - 9,535K
  
  The Art of Unit Testing with examples in .Net, Second Edition
  
  epub - 3,784K
  mobi - 4,356K
  pdf - 9,535K
  
  
  
  Findings
  
  Average Sizes
  
  
  epub - 8,396K
  mobi - 7,717K
  pdf - 11,426K
  
  
  In almost all cases, the epub was the smallest file. Mobi was slightly larger and pdf was the largest. However, the average size of the mobi files were actually smaller overall. This was due to one very large epub file for the HTML5 for .Net Developers book. This book skewed the results since the file was nearly twice the size of the other formats. My best guess is because this file contains many HTML code examples, which may cause problems for the epub format (basically XHTML). Perhaps this is an epub3 format, which supports HTML5 (the book title is a good hint). Again, I did not generate these files, so I am not certain of the reason.
  
  To combat this single anomaly, I decided to remove that book from the averages. When removing the book, the averages of the remaining books looks like so:
  
  Average Sizes (removing HTML5 for .Net Developers - i.e. the anomaly)
  
  
  epub - 5,703K
  mobi - 6,678K
  pdf - 11,134K
  
  
  Take it for what it is worth, but in these findings it suggests that the epub and mobi formats are quite close in size, while the pdf format is nearly double.
  
  Answer to be continued with newer formats and more research.