> I also read somewhere recently a theory that big cars are a way of protecting (even just psychologically) wealthy people from reality as they have to travel through areas of poverty to get from one wealthy pocket to another.

That theory seems to arise from a disgust for the wealthy (rightly or wrongly) as opposed to an earnest evaluation.

I would put money on the theory that big cars can have more nice things in them and are safer, so the rich buy them.