If you use XP, ignore this post.
If you use Vista, this is my only suggestion as to why everything is taking up double space.
Vista uses an automatic-backup system where everything is backed up upon creation and at other times. It's possible that you haven't got a partition on your HDD that stores the backed up data, so it is storing it in the general HDD space, therefore it is saving each file twice on the Hard disk. It should save the original file in the general HDD space, and the back-up in the back-up section.
Mine is partitioned so it does a back-up to there, and my HDD doesn't display the back-up portion when you check your disk space.
It is really difficult to explain so I hope you get at least an idea of what I am trying to say.