The implied algorithm is that you first pick a world size s from some distribution, and then pick an index uniformly from 1..s. This corresponds to the case where there are three separate urns with 1 red, 1000 yellow and 10^6 green balls, and you pick from one of the urns without knowing which one it is.
(I find the second part, picking an index uniformly from 1..s, questionable; but there’s only one sample of evidence with which to determine what the right distribution would be, so there’s little point in speculating on it.)
The implied algorithm is that you first pick a world size s from some distribution, and then pick an index uniformly from 1..s. This corresponds to the case where there are three separate urns with 1 red, 1000 yellow and 10^6 green balls, and you pick from one of the urns without knowing which one it is.
(I find the second part, picking an index uniformly from 1..s, questionable; but there’s only one sample of evidence with which to determine what the right distribution would be, so there’s little point in speculating on it.)