I had this problem too. After editing photos in iPhoto with PS Elements and then viewing them in iPhoto, I could see their sizes (in Info) reduced from 4000x3000 to something smaller, about 1200x900 (though the file SIZE was still around 5 MB), a double click on such photo would only display a fraction of the image, and even more annoyingly, exported photos were only fragments of the image.
I followed Old Toad's advice (seen in another thread) and Repaired iPhoto library, the Rebuild it (neither helped), and then Rebuild the thumbnails - this solved the issue, at least for now.
(As stated by others, after editing with PS Elements, photos cannot be adited with iPhoto. Well, at least they display correctly.)
Gleb G, thank you, thank you, thank you !!! Finally this is THE SOLUTION for this lasting nasty problem !!!! Not repairing the i Photo library, this is not helping, but rebuild the thumbnails, which took all night for my 100.000 pics in the library. After doing this the problem is solved - at least for now - and also the new pictures cropped in CS6 and saved in i Photo show correct size now in iPhoto.