In my opinion the pain that one spank can generate on a set of female buttocks ceteris paribus is the same regardless of the surface area of said buttock.
However in my opinion a spanking can go on for longer on a larger butt before: a) the skin will get numb and not transmit satisfactory amount of pain to the brain, or: b) the bruising will force the spanking to stop.
Therefore generally a more voluptuous females would tend to get longer and/or harder and consequently more tear inducing spankings.
Whether a more muscled or more fatty butt will generate more pain to the owner I am not sure. Probably a more toned, less fatty butt that is harder should be more conducive to pain transmission, however I am not sure on that. | 
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