Why is the iPhoto Library size so much bigger than its contents

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...

This sounds like somehow your iPhoto Library has moved. If you hold down ALT when you click to start iPhoto, the library management dialog comes up. This will tell you where iPhoto is currently located. If you somehow had access to your old HDD, you could see where this was located.

Obviously iPhoto doesn't work without a library, so the library has to be somewhere. I would certainly dig a little to see if you could find it.

_{Unfortunately, I haven't worked with HDD recovery at all. I do know there are companies that do this. You could potentially send it it. I'll update or comment if I do find something.}