As you can see, the whole frame is used. I'm willing to bet that the cels not going all the way to the edge of the frame is why the full screen transfers were tighter. This is why a lot of 1.37 animated films aren't completely open-matte.
My point was that An American Tail is open-matte, but that the widescreen has almost no extra picture on the sides, and I don't know why.