>>5975I assume what he meant to say is that, given one set of parents with two children and one set with 10 children, you are 5 times likelier to be a child of the latter than the child of the former.
Look at it this way: if you grouped together the children of a couple with 10 kids, and five couples who each have two kids, any given kid in the group will have a 1/2 chance of coming from either a 10-kid or a 2-kid household. But as far as individual families go, each kid only has a 1/10 probability of belonging to a 2-child couple, while having a 1/2 probability of coming from the 10-child couple.
The odds of belonging to a 10-child family might be extremely low, but the odds of belonging to it are 5 times those of belonging to any single 2-child family.