iPhoto loves hard disk space like I love Kentucky Fried Chicken! iPhoto will eat your hard disk up if you have a large photo library and do lots of editing. Heres why; iPhoto makes a duplicate copy of your photo on your hard disk every time you make an adjustment to the image.
Once you have your pictures edited the way you like them, you may not want to keep the originals. However deleting them is not as easy as you might think. You can run a script from Terminal that helps keep the iPhoto Library clean, but thats too intimidating for most of our audience. The next best thing is called iPhoto Diet. It might not work as well with the latest version of iPhoto – so use it at your own risk. In my tests with iPhoto 6, it performed perfectly, finding and deleting unwanted duplicates and originals, saving me tons of disk space.
If you just want to remove duplicate images, try Duplicate Annihilator.