The guys over at OXD are ever-impressive. John Greer has posted 2 sample documents originally created in Word 97-2003 format and converted to Office 2007 .docx format.
I believe the point is to provide samples of the file formats and let users see the differences/similarities but you can't help but notice the difference in file size. One of the samples is the full text of Alice's Adventures in Wonderland including illustrations. The Office 2003 version is 528 KB. The Office 2007 version is 276 KB or roughly 52% the original. I'm sure the compression depends on content but this is an amazing example.