I can't give you a definitive answer Paul but have always suspected this may be the case.

I booked a 2 door sub compact car in January through Dollar as I was there by myself for most of the time and refused an upgrade at pick up.

The car I was allocated was actually a 4 door intermediate. I looked around at all the parks for something smaller and never did see anything.

I suppose you could always just book the smallest, refuse the upgrade and see what it is and always go back to upgrade if you need to, but I wasn't bothered how small the car was.

Jill