We Aussies love the feeling of space inside our cars. The rental companies know this and usually stock a great deal more mid-size and large cars than they do the small ones. If you've reserved a small car and there are none when you arrive, then they are obliged to give you a free upgrade. There is always a danger that the small car will be polished up and ready to go, but you can usually just pay for the standard rate for the upgrade on the day. If your little car is the only one left on the lot you can comfort yourself by thinking of the savings you'll make on fuel.