>Cars are a big one.

Yeah, I was thinking about that after I posted. Some go to a minivan, but when you deal with 6+ kids there is a point where you need a second vehicle, but some smaller families already have a 2nd vehicle, one for each working partner. I accept it's a complicated exception but wasn't sure how to quantify it.

Sometimes there will be cases where an extra kid doesn't cost more, until you hit capacity and have to spend twice as much (or thrice if you have an unusually big family).