I suppose a big family could contracept and a smaller one not.

Yes, it’s possible, of course. Remember the anecdote of Bp. Fulton Sheen. He met a couple of newlyweds en route to Europe for their honeymoon at Lourdes. They told him that they planned to pray for a big family and light a candle. The bishop gave them his blessing. Twenty-five years later, the bishop encountered the same couple again on their way to Lourdes. He asked if they were lighting another candle. They replied, “No, Your Excellency, we’ve been blessed with two dozen children, so we’re going to blow the candle out.”