Don't shout "You should never edit photos in iPhoto" as well as "You've destroyed the reversible photo editing capacities!" (that's a bit contradictory, don't you think?). And don't say "you've now saved lower quality images" because they aren't --- rotate an image in iPhoto and it's still exactly the same resolution and quality (and filesize). And there's NO POINT in keeping a sideways image if you can easily substitute it for the correctly-oriented one, plus there's no offered practical solution... meaning your valid points may ignored and destruction & tears follow.
So, there's two sides talking at cross purposes here: One wants to desperately save space, the others warn against destructive changes... I'd say give a bit more detail, and specific warnings instead of a general shout; then each can decide what to do or not.
It's a long post, my excuses, but there's not much more precious than your last decades' photographic memories, so it's worth a bit of time to consider. I don't want to give a short version without motivations, as readers may not see the point and take another step that has bad consequences (or conversely, you can come up with a better idea if you see I overlook something -- it's clearly at your own risk).
First of, since a very long time the iPhoto library isn't a normal folder anymore, but disguised as a package so you cannot as easily rummage through it (yes, just right-click and choose "show contents"): For the very good reason that loads of people have been hand-pruning in there destroying their whole database, and earning thousands upon thousands for data recovery specialists and mac gurus who spend days undoing the mess. [So go ahead, mess in the Library without thinking; maybe I'll make some money off you later on; if not and you lose all your mementos, too bad but no skin off my nose... OK, that wasn't meant seriously.]
So, I like the "cp -rf modified/* originals/" idea very much. Let's do it orderly:
[1] Make backup on external HD or USB stick if feasible (and check it works; any untested backup is almost as good as no backup). If you ruin things later on, you can quickly painlessly revert to whatever you started with. [My partner's library was 100GB for only about 18500 images from the last 10years --- starting with 0.6Mpx images to recent 5--8Mpx ones... so clearly something badly wrong.]
[2] Maybe use something like "Duplicate Cleaner for iPhoto", after careful checking: Repeated images you should put in different albums, not copy the images to different events. This step will mess you up, if you've mishandled the format like that: e.g. if you have a "my best images" event ahead of the others, the photo version in the "best" folder remains while that in the actual event disappears. [However, 30% of my partners pictures turned out to be duplicates --- up to 6fold --- and were accidental copies of entire events, not pick-and-mix duplications to make e.g. a calendar or digital photoframe selection... Still didn't explain the size problem, deleting those saved only 5GB or so.]
[3] If you know that the only DESIRED changes you ever made in iPhoto are rotations, then:
[3a] "Revert to originals" as said before (throwing away all changes, going losslessly back to your originals),
[3a'] Check this has happened, the "modified" folders are all empty,
[3b] Go through all your iPhotos, scan manually through them and correctly rotate whatever you want rotated,
[3c] Now do the "cp -rf modified/* originals/" to consolidate your rotations, with the "revert to originals" and empty the iPhoto trash (not your mac trash).
Step 3a is for a complication that hasn't been mentioned in this thread up to now: Whenever you click&drag on a photo in iPhoto, it thinks you may want to do some cropping or other alteration (redeye?) -- and it generates a "modified" version. Or you accidentally rotate, so you rotate it back to what it was. Probably there's other similar causes. About 80% of the "modifications" in my Library seem to be that: I don't see a difference original/copy... So these are changes I definitely want to discard.
Clearly 3b+3c is better to do here in iPhoto than in an external editor as (a) doesn't cost money/time to install such an editor and find out how it works (depending on quality settings, your rotated image may be a smaller filesize of lower quality, or larger of similar quality --- iPhoto rotation does exactly what it says no further compression/alteration), and (b) because otherwise you have to re-upload all affected images after editing them externally, putting them back into whatever events and albums they come from (a risky task), and then manually delete all the unrotated originals.
A further source of duplication/overload is if you add in non-JPG images, maybe dependent on Preferences, maybe only for specific formats (I don't know), then iPhoto makes a JPG copy keeping the original. [In my partner's library, there was a 200-picture event that was all in Canon RAW from a visiting friend's camera --- 10MB per image for simple 6Mpx images; it can be worse my mother came with 25MB TIFF files for 5Mpx images.] That you have to explore, it shouldn't happen too often.
Remember, we're trying to deal with an existing mess, not create a workflow for future things. And we're not speaking serious amateurs (who want to have their RAW versions to get the maximum out of their photo manipulation software and --constantly improving-- skills) nor professionals (who create GIGABYTES per event, e.g. marriage or fashion shoot). For any new images, it's indeed far better to first run all images through e.g. lemkesoft's GraphicConverter, deleting all the non-memorable images and subsequently use its "convert&modify" to copy all these new images at a set JPG quality (say 85% --- depends very much on your camera). Here you MUST be familiar with your camera + your software to know the "sweet spot" / "most bang for the buck" quality setting. Clearly you're not going to do the compression on your marriage photos or those of your newborn.
[iPhone/iPad optics are so poor they don't justify 5Mpx images IMO --- a 10y old 2Mpx Leica makes FAR BETTER images, but it's bulkier and you don't have it with you always; that's another discussion. In short, I routinely scale them down in the "conv&mod" batch to 3mpx -- unless taken with external "clip-on" optics.]
[In my partner's library, the mentioned 200-image event had only about 20 memorable images, so that 2GB(!) event (in Canon RAWs) was reduced to some 25MB for the 20 'keepers' (in JPGs). Anything without serious lenses and mechanical zooms just isn't really worth it... Actually, reducing settings from 5Mpix to 3Mpix can seriously improve image quality at low-light, in a way that post-production on them cannot (easily?) match.]
[A further thing in my partner's library is that about 38GB of its 100GB is taken up by the "ipod previews" --- it seems there's 500KB per low-resolution version of all iPhoto images. The whole library is accumulated stuff over 5 iPhoto generations, and three computers.]
In any case: Think before you act, as storage space gets cheaper every year and reducing your images quality now can never be undone. You could keep your libary as-is on a spacious HD (desktop, external, whatever) and make a "all images compressed at 85% and scaled to maximally 4Mpx" iPhoto library ---you know iPhoto can have multiple libraries to be chosen at startup?--- and use that smallish library to copy to your secondary laptops/ipads/ipods. [My partner's iPhoto is on a iMac, Macbook Air, iPad Mini Air, iPad 3rd gen, iPhone, and iPod Video --- several of whom are choked full of images, "I should have bought the 128GB instead of 64GB!", which would cost hundreds of dollars in upgrades.]
