What you’re asking is a hell of automation and it can’t be done “just” like that.
I suggest you try to make a combination of Automator and Smart Albums on iPhoto. Things you want to know about iTunes: EXIF is not preserved when Synced to iPhone through iTunes which is part of your requirements.
You could use a Smart Album in iPhoto and Flag the pictures you want to transfer then create another Smart Album with those flagged pictures and only transfer those (or vice versa). Use Keywords to your advantage. Consider a nice app like Keyword Manager for iPhoto to help you with that.
I have no idea why iPhoto makes you nervous, but you should be pointing at iTunes for that…
iPhoto is perfectly suitable for picture maintenance and sorting. You can later use automator to resize them and do fancy things (even tagging!). The same can be said about Aperture.
Your workflow should be, download to iPhoto automatically. Classify, Tag, Delete Bad ones, Adjust, Edit, and transfer to your devices for later viewing (and backup). If you just want to have a backup up of what you do, let Time Machine do it’s work with your iPhoto Library or add another step, use Image Import (or Cameras) to organize, then add to iPhoto for treatment and later transfer to your device(s). The original imported folder will be untouched and, eventually backed up by Time Machine too.
You could probably do a database repair. Hold down command+alt+shift and click on iPhoto's icon to start it. You should be presented with a window and multiple commands. The rebuild database and rebuild thumbnails options should help.
I know that when you sync photos to an iOS device, photos are scaled downed then synced to the device, and these scaled down photos are stored within iPhoto's library. But it wouldn't explain the library doubling, though... strange.
Make sure you have a backup of the library before doing this operation, you never know...
Best Answer
There is no way to do this in iPhoto, as what you're doing is destroying data, and iPhoto is built to expressly avoid this.
So, File -> Export, set the Kind to Jpeg and then choose from the Quality setting as appropriate. Export the images to a folder on the desktop.
Trash the images from iPhoto.
Import the exported ones.
Note some things 1: You lose your lossless editing history and Faces data at the very least. If any of the images are used in Books, Slideshows etc then these will have to be remade.
If space is an issue, have you considered moving the Library to a larger disk - an external, for instance? It's easy:
Make sure the drive is formatted Mac OS Extended (Journaled)