It’s basic math.
Think about the ratio of surface area to volume. Take a 10 by 10 by 10 foot cube. It has 60 square feet of exterior surface, and 1000 cubic feet. In a house that surface leaks heat.
Now add 10 feet to one dimension. It will have 100 square feet of surface, and enclose 2000 cubic feet. So the thermal emission surface went from 60/1000 (.06) to 100/2000 (.05) – so the increase in size made it 1/6 more efficient.
The numbers get better when you maximize volume to exterior surface.