It depends on what you call household size, Flickety.
Do you believe that anyone should be forced to live in a one bedroomed place just because their children have left home?
Do you believe that a couple should be forced to live in a single bedroomed flat, with no spare room for guests or office or grandchildren to stay when visiting?
Do you believe that if you have two children of the same sex, then they should be forced to share a bedroom until they are sixteen, so you can only have a two bedroomed house or flat?
When the older one reaches sixteen, then the family can move to a three bedroomed place, but only until one of the children leaves home. Then it's back to two bedrooms.
Of course, if the children are opposite sex, then you can have a three bedroomed place once the older one is ten, but you still have to move to two bedrooms when one of them leaves home.
It's just punishment.