I'm not the OP, but DH and I are both from almost equally big families, and his mother is the eldest of 13. In our cases, it was the lack of ability to avoid conception short of celibacy, given the availability of contraception in Ireland and the fact that 'artificial' contraception was not allowed in Catholicism -- the two facts closely meshed, obviously, given the close intersection of church and state in Ireland until fairly recently.
DH and I have one child by choice.