I usually try to decline upgrades, and depending on inventory, sometimes they find a smaller car, and sometimes they don't. I get the feeling that at smaller lots, there are some cars they don't want to rent for whatever reason.

Anyway, there is never harm in asking. When they've given me a large car I don't want, I've *) refused it and gotten another one, *) waited, *) taken it and replaced it the next day after calling, *) put up with it.

Another option is to go to another agency, but that has cost implications.