Hi Chris,
I'm sure the experts will chime in anytime now, but this is my understanding of the difference...
It has to do with the area each pixel covers (pixel size), you may have the same number of pixels but a cropped sensor will have a smaller effective pixel size, effectively increasing pixel density, making the image appear larger for a given frame.
I'm not pursuing astrophotography but have been around long enough to gleaned a few concepts. I think this same principle applies to the example you gave.
All I can add now is that I hope I'm close to teh mark and haven't create a pool of confusion...
