You’re right that it’s a rule of thumb rather than a real correlation. Since heating and cooling costs are high enough for most people to drown out other use, it’s a reasonably good approximation, especially since there are a million other variables in play as well (gas vs. electric appliances, ranch-style versus multiple floor housing, outdoor pools, etc).

I’m not sure the building size directly correlates with the usage though. If I lived in a 1500 sq ft home instead of my ~450 sq ft apartment, the heating/cooling would go up, but cooking, computers, washing clothes, etc. all stay the same.