A difference between iPhoto and Aperture that results in Aperture taking up "less" space than iPhoto is that when you edit photo's in iPhoto, the program creates a copy to edit, thus you have two copies of the same photo. Aperture doesn't physically edit the photo or even a copy of the photo. It just writes what you want to do to the photo and generates a preview of what it looks like, and in order to get the edited photo, you need to export it, therefore making a real copy of it.