As I said in response to Gwern's comment, there is uncertainty over rates of expropriation/loss, and the expected value disproportionately comes from the possibility of low loss rates. That is why Robin talks about 1/1000, he's raising the possibility that the legal order will be such as to sustain great growth, and the laws of physics will allow unreasonably large populations or wealth.

Now, it is still a pretty questionable comparison, because there are plenty of other possibilities for mega-influence, like changing the probability that such compounding can take place (and isn't pre-empted by expropriation, nuclear war, etc).