"A result of this is that the usual estimate is that 80 per cent of the women who have ever lived had children while only 40 per cent of the men did. We see echoes of this in even modern populations when they're put under stress: the ratio of boys born to girls falls. In famine, for example, a child is likely to be undersized as a result: but it's going to be easier for an undersized woman to have those desired grandchildren than a runt of a boy."
I think that the author and I live in different universes... In MY universe, the number of (human) males that have ever conceived children is something Very Close to 0%, where in the author's universe it seems to be about 40%. And in MY universe that percentage does not vary much, if at all, even if the foetus is under-sized... Males conceiving and carrying even runts to full term is still Pretty Close to 0%.