Thank you for your reply, Leonie, and you are correct that the 16x9 is simply a cropped version of the 4x3 image. However, the camera is not arbitrarily choosing where to place the rectangle in the cropping process. I choose that and I am the one in control of the cropping when I shoot the picture. I see the exact 16x9 image and compose it as I wish when I am shooting, so there is no need to have the 4x3 remain on my camera since I would never choose to compose it differently than I did when I took the picture the way I wanted in the first place. But apparently Apple has decided to use up our storage space for no reason,
and not even give us the option to do otherwise.
A related problem is that I did a test photo today to confirm my suspicion that the AAE files are appearing, whether or not I have done any editing on the picture. I took a test photo, and did no editing whatsoever. For that one picture, when it came time to transfer it to my computer, I had to deal with four saved files rather than the one that I wanted. It saved the 16x9, a 4x3, and an AAE for each of them! Transferring files to my computer has become hugely time-consuming because of all these useless extra files for every single shot I take. When you take many hundreds of pictures, that is a substantial waste of time.