I'm afraid I can't tell you why it happens, but my roommate once had the exact same problem with his couple-page document, only the resulting PDF was something ungodly like 30 MB. He gave me the docx, I simply opened and saved it as PDF using Word for Windows (using the same settings), and the resulting PDF was less than 1 MB. He said the PDFs looked identical and the text was equally selectable. Go figure.
So if you have a friend with Windows and Word 2010 or 2007, you might be able to save yourself a lot of time trying to work around it on your Mac and move on with your life. (I wouldn't say that if it were a more regular occurrence, particularly since I'm now a Mac user myself.) :-)
Alternatively, other things to try:
- Use Mac OS X's ability to "print" arbitrary things to PDF. To do so, in Word, press Command+P, then clck the "PDF" drop-down menu in the lower left corner of the dialog and select "Save as PDF". I don't believe this performed any better when my roommate tried it, but perhaps it would in the case of your document.
- If you have a copy of Adobe Acrobat, use it for the conversion. If any piece of software would "get it right", I'd think that would be it.