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.
xlsx) are actually zip compressed files so they will be smaller then some other native formats.