If you are a Blue Chipper and ask nicely, you will from time to time end up in a fullsize no matter what you have rented. At some stations, the planning is so poor that if you arrive at a busy time, the car assignment almost seems to be random (DCA comes to mind) and you can end up with anything no matter what you have reserved. At well run stations, they will almost always have exactly what you have rented and there is no movement, no matter how regular a customer you are and no matter if you and the staff have been on first name bases for years (SDF comes to mind).
Oddly enough, one of the last times that I rented from Thrifty in South Florida (with a res for a fullsize), the only cars left on the lot when I got there were some convertibles. This was the only time that I have been upgraded to a convertible in many years of doing business with them.