I use a commercial app called CleanMyMac 2 which can scan your iPhoto libraries and safely remove many gigabytes of files that you don't need. It does this by, among other things, looking for pictures that you have cropped, and identifying the uncropped originals and asking you if you want to delete them.
From their web page:
Find what you can’t see
Your iPhoto library is a lot larger than you
think it is. Every time a photo is cropped, rotated or changed, its
modified file is stored in the iPhoto library, while the original is
kept hidden. CleanMyMac 2 locates all of those hidden originals.
Auto or manual cleaning
You can let CleanMyMac 2 automatically select
which invisible originals are safe to remove. Or you can manually
review each image and remove only those you no longer want. How much
space you clear is up to you!
CleanMyMac costs US $40. It can also free up a great deal of space on your hard drive by cleaning up and deleting many other kinds of files besides iPhoto files. You can set it up to run automatically at regular intervals and report to you what files it has found that you can choose to delete.
Update: CleanMyMac 2.1.0 now supports iPhoto 9.5 and Mac OS X 10.9 Mavericks.