The size of a PDF is determined by much more than the amount of text. Any graphic elements (not necessarily a picture or illustration) will increase the file size. There have been reports that using the PDF option in the print dialog instead of Save as PDF produces a smaller file:
For More Information see: My pdf files have inflated by 20 times since converting my existing file from version 6 to version...
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html