I had to convert 17 PDFs to the Windows WMF format for inclusion in an Excel worksheet. Illustrator CS3 on OS X does a good job of exporting to the WMF format but a 40KB PDF ballooned into a 348KB WMF.
I reopened the WMF in Illustrator and re-exported the WMF as another WMF. The newly re-exported WMF had a file size of of just 76KB.
My knowledge of Illustrator is very limited so I have no explanations, but it does seem like a useful technique.