Why does Distiller double my file size?

I am using Acrobat Distiller to create a secured PDF (InDesign export to PDF --> Save As *.ps --> process through Acrobat Distiller --> secured PDF), but it is doubling my file size despite the fact that the settings are the same as my export settings.  (I have to use distiller to add security because adding security directly to the PDF file changes the version from 5.x to 7.x and I need it to remain 5.x for server/license settings.)