[I replied to the wrong comment, so I copy my earlier answer here, at its right place, sorry.]
I assumed they were just continuing a trend. But I looked at the US 1980 census and was surprised to find that the number of persons per household almost didn't change. 2.67 then vs 2.62 now.
Meanwhile in France, it dropped from 2.75 to 2.2, so I thought it was the same in the USA, hence my hypothesis. But it appears it isn't.
I assumed they were just continuing a trend. But I looked at the US 1980 census and was surprised to find that the number of persons per household almost didn't change. 2.67 then vs 2.62 now.
Meanwhile in France, it dropped from 2.75 to 2.2, so I thought it was the same in the USA, hence my hypothesis. But it appears it isn't.