Good Roman Catholics do not need to breed like "rabbits", the Pope has said, but should practice "responsible" parenting instead.
Pope Francis spoke as he returned from the Philippines, where he met former street children abandoned by parents unable to afford to care for them.
Standing firm against artificial birth control, he said new life was "part of the sacrament of marriage".
But he said population experts advised three children per family.
Pope Francis raised eyebrows last week when, in the aftermath of the Charlie Hebdo killings in Paris, he said it was wrong to provoke others by insulting their religion.
He told journalists as a joke that his assistant could expect a punch if he ''cursed his mother''. At the same time, the Pope defended freedom of expression.
'Excuse my expression'
Speaking to journalists while heading back to Rome from the Philippines on Monday, Pope Francis was asked what he would say to families who had more children than they could afford because the Church forbids artificial contraception.
He replied with an unexpected turn of phrase: "Some people think that - excuse my expression here - that in order to be good Catholics we have to be like rabbits."
"No. Parenthood is about being responsible. This is clear."
The Pope said he knew many ways allowed by the Catholic Church that could ensure families only had as many children as they wished.
He cited the case of one woman he had met who had had seven children by Caesarean section and was expecting her eighth - a pregnancy he said was irresponsible.
"She said, 'I trust in God.' But God gave us the means to be responsible," the Pope said.
To a separate question, the Pope said that most importantly, no outside institution should impose its views on families.
Progressive, Western ideas about birth control and gay rights were increasingly being imposed by groups, institutions or nations there, often as a condition for development aid, he said.
"Every people deserves to conserve its identity without being ideologically colonised," the Pope said.
During his trip to the Philippines the Pope defended traditional Vatican teaching, which opposes artificial contraception.
On Sunday, an estimated six million people attended an outdoor mass he celebrated in the capital, Manila.(BBC)