iPhoto '08 creating modified files automatically?

I want to free up hard drive space. I don't use iPhoto to alter images, so I do not understand why modified images are self-creating. I am also not touching any actual image files (outside of iPhoto) aside from looking at them.

1. I open iPhoto Library with "Show Package Contents" and open up two Finder windows, one with Modified, and one with Original. For example, I'll locate a modified photo in "2009 > Apr 18, 2009 > DSC_8157.jpg"

2. I find the same photo in the Originals folder and compare the two. Example: Original file is 4.8 MB. Modified file is 5.2 MB. Visual inspection of both images leads me to believe they are both the same.

3. The one thing I have noticed is all modified images are vertical (I rotated the camera when I photographed them).

4. In iPhoto, I select the photo, right-click, and choose "Revert to original". I click "OK" when the warning pops up, and the file is apparently restored. Looking at the Finder windows, I see the file deleted. But it returns shortly thereafter.

So, in writing this, I discovered the answer to my question: it must be the vertical images that are being modified. But this is essentially duplicating the image (in a larger size, for some reason) and filling up my hard drive.

Is there nothing I can do?