People want big things and climate only 'cares' about the emissions, so focus on the energy bill not on the size.
To the extent those are the same, the market forces work out the same anyway.
To the extent that they are different, focusing on the wrong thing means Goodheart's law will bite, and you'll end up with small dirty vehicles/homes instead of encouraging innovation that would allow big clean vehicles/homes.
Both the energy and materials required to manufacture and transport the car to the dealership are dramatically impacted by size. Between a light compact car and a huge SUV there is a way bigger difference in those costs that just weight would suggest.
There's also other factors such as microplastics: most of those on our planet are not coming from plastic straws but tire wear. Bigger cars require bigger tires which will wear more due to weight and higher stress.
We just can't pretend that the way we consume does not impact the planet.
I'm not against big cars. But they should get taxed more, as in many parts of the world, whereas in US legislation which focused on the ratio between weight and fuel consumption basically made it more efficient to just build bigger trucks.
1. Which was bigger, and which was the greater source of emissions: a Ford Prefect, or one of Columbus' galleons?
2. I own a 38 m^2 apartment in the UK, and am buying a 107 m^2 house in Germany. The big one has approximately half the annual energy requirements of the small one.
(That's raw power requirements, not CO2 etc., so it doesn't account for e.g. PV being used to supply that power).
3. Given the need to reduce greenhouse gas emissions by 99.9% in order to stabilise the temperature (because of how long CO2 stays in the air), if it was as simple as "mass ∝ greenhouse gas emissions", then simply downsizing cars would not be sufficient — we'd have difficulty wearing clothes at that ratio, as even just your shoes wearing out becomes significant given the kinds of changes we want.
It's such an enormous change that we can only solve it by a near total replacement for every part of almost all our industrial processes, and once we do that replacement, then we have to look again at what is possible, and decide from that what rules we need for houses and transport and everything else.
If all the steel and aluminium, all the batteries and plastics, are zero carbon at that point, then it doesn't matter how much of them you use — zero CO2 per kg is still zero total, no matter if the vehicle mass is 100 kg or 10,000 kg.
To the extent those are the same, the market forces work out the same anyway.
To the extent that they are different, focusing on the wrong thing means Goodheart's law will bite, and you'll end up with small dirty vehicles/homes instead of encouraging innovation that would allow big clean vehicles/homes.