Jennifer Fulwiler has a decent bit in NCR today called "Why My Big Family Is Not Overpopulating The Earth." Personally, I have always liked Simcha's writing on her huge green family.
I have a theory that the frequency at which you get chewed out for the carbon footprint of your children increases exponentially by the number of kids:
𝝓(n) = 𝝓1 ek(x,y)n
where 𝝓(n) is the frequency of being chewed out for having n children, 𝝓1 is the frequency of being chewed out for having 1 child, and the growth constant k depends on where you live.
(The value of k is larger in San Francisco than in Peoria. Let's be charitable; maybe if you spent all your time in San Francisco, you'd probably think the world had too many people too.)
So, in many places, one to three kids hardly raises an eyebrow. At four kids, I only get a small fraction of the negative comments that a mother of five would have, i.e., 1/ek(Minneapolis).
Anyway, I often appeal to diversity. "It takes all sizes of families to make a healthy, sustainable population distribution!" I say cheerfully.
I don't generally show them pictures of bell curves, although I'm tempted to.