STDUViewer works with epub, pdf, djvu and many other e-book and some picture formats.
It has 10MB and doesn't even have to be installed (there is choice to download installer or *.zip with portable version)
http://www.stdutility.com/stduviewer.html
DISCLAIMER: I am un-affiliated. I just found the software when I was looking for a bare-bone, small, visually un-cluttered viewer to display some relatively niche e-book formats.
Normally I am big fan of Calibre and its viewer.