Good detective work! I have to agree about the defragging of the HD often
. I have both RFB and OM installed (presently playing a Drumbeat campaign), and don't seem to have large save files. If I do, I've never noticed because the games seem to save and reload fairly quickly.
As you pointed out, HD maintainance is key! I use Perfectdisk myself, and alsoCcleaner. I run them both every time I play!