Isn't this also due to the Prolactin receptors?  That once you breastfeed
one child the next time around your body replies more quickly to prolactin,
and so on.   With each subsequent child your body becomes more responsive
to Prolactin.  This is why older women who have had a lot of children can
produce as much milk as a young woman - even though her Prolactin levels
are lower.  This has also been used as an explanation for low supply  in
mothers of premies who weren't able to get these receptors going.
This is why it is so important to start feeding as soon as
possible after the birth.

Nofia