There is a principle in architecture known as the "golden ratio" or "golden mean."
Basically, this principle states that there is a certain mathematical relationship between the width and height of an object that appears "aesthetically pleasing" to the eye.
Architects and industrial designers have striven to proportion their works in accordance with the Golden Ratio at least since the Renaissance era.
Granted, this does apply more to buildings and structures than to industrial design, but the same general aesthetic principles do follow.
If you were to examine a given object and it seems to be innately attractive to you, the likelihood is great that its design is proportioned in accord with the Golden Ratio.
The 516-D80 and Classic series Kirbys, I believe, more follow the Golden Ratio than any other Kirby models. The proportions of the motor housings not only are perfectly balanced, ratio-wise, but also beautifully designed.
Then you have the Bison.
Its design from nearly every angle flies in the face of the golden ratio, especially from this perspective:
The lines of the body are jangled, disproportionate, angles oddly jutting out in visual dissonance. It looks klunky and disorganized. And indeed, it is. The inventor of the Bison may have had some good ideas but he had no concept whatsoever of pleasing industrial design and the golden ratio!
See link for more info if you're interested.
http://en.wikipedia.org/wiki/Golden_ratio