Not to get into an argument on ohms law as it applys to DC motors, I would suggest there are several things going on here:
1) When supplied with 12 volts, the 6 volt starter motor's initial impact with the ring gear is going to be significantly greater. I would attribute this impact loading, as most responsibile for breaking parts, that you have observed.
2) An estimate of the additional power input to the ring gear can be obtained from the engine's cranking speed. If the cranking speed doubles, assuming all else constant, then the starter's HP has doubled...approximately. I don't think you will see four times the cranking speed simply by switching to 12 volts - I would attribute this to the back EMF effect.
Summary: If the starter motor is accelerated to twice the speed before engaging the ring gear it's rotational energy is increased by a factor of four (2 squared). If we assume the starter motor has twice the power output, then the load is only increased by a factor of two, during actual engine cranking. However, if one were to simply combine the impact load and the power load, then you could say the actual parts involved see a load increase of eight.
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