Is there any way to add support for epub & mobi to the default document viewer in Ubuntu?
Calibre is an overkill for me there is any other small and light software out there?
You can use FBReader. FBReader is a multi-platform e-book reader.
supports several open e-book formats: fb2, html, chm, plucker, palmdoc, ztxt, tcr (psion text), rtf, oeb, openreader, non-DRM'ed mobipocket, plain text, epub, eReader1
You can download and install from the Ubuntu Software Center, or click the image below: