Mike, I know this topic of fit to paper is well covered but I found this interesting, maybe it is already mentioned and I missed it. I defined a 12x12 media size and it was approx 11.7x11.7 printable. Auto-crop is off on the following, I placed a 4x6 image and selected fit to page and it gave me the expanded image filled to the vertical edges with approx 2 inch white space on the top and bottom which is what I expected. I placed another 4x6 and selected fit to paper, it gave the same image as the fit to page ( two inch white space top and bottom). I selected fit to paper again on the same image and it filled the entire printable area.
Basically, it took two clicks to fit to paper. Perhaps it is the 12x12 format that caused this but it would be confusing to someone who is expecting it to fill the paper on the first click. It is as though it needed an intermediary crop to fit to paper.