> How is that a "larger change" for country A, just because they have larger cars?
We are never actually going to get to zero carbon emissions. There are many industrial operations that will emit CO2. The hope is that we can plant enough trees that the sucked CO2 is equal to those non-replaceable emissions to get to net zero.
In your specific case of cars, the production of cars does emit CO2. And production of larger cars does emit more CO2. Whether the electricity used for them is from solar, wind or nuclear, there are emissions associated with the production of panels, wind turbines or nuclear power plants. Constructing and maintaining roads also emits CO2, and larger cars require thicker and broader roads. So, a country with smaller cars will emit less CO2 ceteris paribus. Hence it is indeed better if the average car size is smaller.
Or if people start owning fewer cars and take the bus instead. And of course more people in the large-car country will have to switch to the bus than in the small-car country for the same reduction in emissions.