I just discovered something interesting: If I sync the exact same photos and events from Aperture instead of iPhoto to my iPad, then I gain a lot of free space on my iPad. I sync 1500 photos to my iPad. These photos take about 3GB when synced from iPhoto, but when synced from Aperture they need only 800MB. This is also the case when syncing me iphone 4. Picture quality is exactly the same on my iPad, so I don't see why I should use iPhoto instead to sync. Have you also seen such a behavior?