| Andy, the 996 has a standard ratio gearbox while the 999R has a close ratio, so 14/39 gearing on a 996 is equivalent to a 15/38 on a 999R.
John Hackett's matchstick analogy is good way to think about the larger bending moments in the 525 pins but chains don't fail in this manner. The most common failure mechanism is that side plate holes elongate and distort allowing the peened-over pin ends to pull away from the outer link plates. This is also the main reason why clip-type master links only have 60-70% of the strength of a machine-peened link.
The reason for the longer 525 pins is to gain a larger contact area and lower load distribution between the roller and the wider 525 sprocket face, thereby reducing the wear rate all other things being equal. | 
Shazaam!
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