Did you always want a big family? If not, when did you change your mind?
No. There was actually a time in my life when I thought, "one baby is good". And then I had my second, which secured my "normal" status and we didn't think much more about it. When we were expecting our third, people were surprised and we felt like that would probably be our last baby.
Then a friend casually asked me one day, "So, have you ever considered letting God be sovereign over your womb?" My first thought was that the question she asked was a silly one. "After all", I argued, "I don't just 'let God be sovereign' in other areas of my health, which is why I take medicine for sickness". I wasn't realizing what a vast difference the treatment of sickness, which is not God's creation, and the prevention of a healthy condition was.
After we weren't able to biblically defend our opposition to the regular use of birth control, we gave it to the Lord, and then He made it more and more clear to us the blessing of children and the benefits of recognizing His sovereignty.