I guess aspect ratio comes in to this. You never mentioned what camera you were working with, but if it is a point and shoot, many use an aspect ratio of 4:3, and that does not quite fit a 3:2 aspect ratio paper like 4"X6"...
For printing, Qimage does a great job. I've used it for many years, and it gives you all the flexibility you want... For example it will zoom and crop to fill the whole paper size if you want, or leave white borders to be cut later to fill the paper size if the aspect ratio does not fill the entire paper size.
If you are interested, look here:
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Dick