You got that backward, more people would have to switch in the small car country. Confusingly, that could be an argument for prioritising change in the large car country - if the smaller number of people made the change easier - but I don't think it would, anyway.

You originally said "a larger change to the structure of society" is required if a country emits more carbon per person, and I said this was balls because America could be doing it in exactly the same structural way as China, only more of it. For another example, suppose China is exactly identical to America apart from having an extra 1 billion very poor people who emit no greenhouse gasses at all. Now is there any reason the burden of responsibility, or priority for change, falls on America instead of on that version of China? I say no, that would make no sense.