I have noticed only now that my Word 2010 (docx) documents that are just a single page long and include a simple WMF vector graphic and a bit of text are almost 1 MB large. The Word document is only 50 kB and a PDF file created with Bullzip PDF printer is about the same size. So what is Microsoft writing into the other 950 kB?
Update: As I keep getting answers recently that all do not apply, I'd like to save you the work. The issue has gone away after using Windows 7 instead of XP (which I did over a year ago). Something doesn't seem to be supported on the old system, I suspect it's some font subsetting or so. Also I cannot try your suggestions because the issue does not exist anymore. So I'm not able to accept answers to this.