OK, I think I understand what AlanF has described but it doesn't seem overly useful in telling me what percentage of the original frame the displayed portion is.
For illustration here is a shot that was RAW 5472 X 3648 and 23815 KB. I cropped the bird as shown and it indicated 3041 X 1733. If I posted it as 3041 X 1733, which is likely too large, is that a 100% crop? What is it as posted, 1520 X 867?
It seems to me it would be more userful or interesting to have an idea how this filled the original frame, no?