I have made pictures on high resolution with a Canon 100D camera, which means the pictures are 5-6 MB.
Are the photos raw photos or jpeg? When you edit raw photos in iPhoto on your iPad, the raw image will not be processed, only the embedded jpeg. That will reduce the resolution of the image considerably.
But when I edit them on my Ipad2 in Iphoto it decreases the size and resolution of the pictures to about 0.5MB
How do you save he edited version after editing? That will determine, if the image will be scaled down,
For a full-resolution edit you need to share the edited photo to the Camera Roll,
see this help page: Edit a photo
If you edit a photo in the iPhoto Camera Roll album, the edited photo automatically appears in the Camera Roll on your iOS device, at the resolution of your device. If you want a full-resolution version of the edited photo sent to the Camera Roll, tap 
and tap Camera Roll. The full-resolution photo appears in the Camera Roll, and is transferred to your computer when you sync the Camera Roll with your computer.
And does this also happen with Iphoto on a macbook air?
iPhoto on a Mac has a lossless workflow. iPhoto will always store the original image file that you imported and compute an edited version from this stored original. The quality of the photo you will see outside iPhoto will depend on the methode you use for sharing. You can export at the original size or scaled down, depending on the purpose you want teh photo for, small for email and web presentation, full versions for printing, for example.
