A collection of sources of waste.
Much like how a car is inherently inefficient due to it’s own weight (Car Inefficiency from Weight) - you heat and cool your entire house so that you are a good temperature. You could wear more clothes (or fewer) and use less energy. OR you could just not have a ton of extra space in your house. Vaulted ceilings are nice, but hold a lot of air that needs conditioned.
This is one of the benefits of Tiny Houses.