I just read that more families are buying SUVs even if the number of children per family hasn't gone up.
I don't know about the USA but here in Canada there are a lot of separated couples with children. When you are separated and have part-time custody of your childrens, you often end up with a new partner in the same situation.
So now these 2 parents don't have more than 2 childrens each for example, but they alternate between one week with no children, and one week with 4 childrens. Then you need a vehicle which can hold 6 passengers and their luggage. So you end up with a minivan of an SUV with 3 rows.
The total number of childrens has not increased, but since a lot of families have share custody of those childrens the consequence is that you have reconstituted families which are bigger one week and smaller the next week.
Same thing with houses, we need 4 bedrooms and 2 bathrooms even if half the time we're only 2 in the house.