How can the size of a .blend file be reduced? I noticed that a .blend containing a single material downloaded from Open Material Repository has only 63KB. 

The default scene saved with compression already uses 447KB of disk space. How can files with less than 100KB be created?
There is an option when you save a .blend file, to compress it (uses a basic zip compression iirc).

There’s also an option to always do that by default, in user preferences, File tab, Save &amp; Load section.