iPhoto Library Size Doubled when Synced with iPad
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...