> How can size not be an issue behind emissions?
There are three ways to look at this.
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.