We have found in the past that with most car firms at Orlando airport, but especially Dollar, the smaller the car you've ordered, the more likely you are to get a free upgrade.
Once where they tried to offer us a "special offer" to upgrade at about half the normal price, but we refused as we didn't want a larger car (my wife drives a compact sized one in the UK, so is used to it). Yet when we went to pick it up, it was the upgraded one anyway - I guess they realised they didn't have our car size in, so they thought they'd try to at least get some money out of us!
Heck, last year we had to go back to the desk and ask to be DOWN-graded! When we got to the parking lot, we found the car they'd given us was about the size of an M-1 Abrahams tank! Couldn't see the back of the car when reversing was a bit of a worry, so got them to give us something a tad smaller...