Different results from different size images with same physical parameters

I have been noticed that there is an inconsistency between the results that I create on different sized images - e.g., the results differ if I create 20x20 or 50x50 images although I have used all the same parameters. That obviously should not happen.... The differences seem to be most pronounced in the ``scattering'' component around the central spot. This is manifested in the image below which shows:

Top left: 20x20 cut from the larger 50x50 image

Top right: 20x20 image

Bottom left: full 50x50 image

Bottom right: (20x20 cut from the larger 50x50 image-20x20 image)/(20x20 image)

This ticket is to investigate and fix this problem.