For most users, I think the zero padding is merely an implementation detail about how we handle interpolated images.  We haven't mentioned the zero-padding in any of the other demo scripts, even though it was used the other places we use RealGalaxy.  So I think going into detail about it here would just be confusing.  

There is some discussion of it in the Config Documentation for the `pad_factor` option, and more in the docstring for InterpolatedImage and RealGalaxy.  I suspect these are the right places to go into detail about this behavior.