Broken: 2.79 (Tried 2.8 as well and I have the same issue)
When I take an image in the compositor and scale it up 2x or whatever and then separate the RGBA and dilate/erode the alpha and use that to set the alpha on the image, it only works within the size of the original image and not the scaled up image. Also even if I use the R, G, or B, it still only uses the size of the original image and not that of the scaled up one.
Cropped if I try to mess with the alpha after a transform.
Uncropped if I mute my changes to the alpha.
Also I believe that if you scale down