I know what you are talking about. My partner has the 4 and all his photos are ever so slightly zoomed in by default when browsing. If you pinch to zoom out you can see the full size with black boarders. On the 5 the black boarder are there by default. The only thing I can think of is that if they zoomed all the pictures in to fill the screen like they do on the 4 is would crop too much out because of the large screen. Still they could have zoomed it in the same amount they do on the 4 so the bars appear smaller.
Came in here to post this. No one else before him understood the OP.