Yeah, we've been through EXACTLY the same thing with Dollar twice before now. They spent ages trying to get my wife to upgrade our booked Economy car:
Agent - "it's not very roomy"
DW - "there's only the two of us"
Agent - "there's not enough space in the trunk for all those suitcases"
DW - "we'll only have them in the car on our journeys to and from the airport"
Agent - "a bigger car will come with all these impressive features"
DW - "I drive a car smaller than your Economy size back home, a bigger car would be like driving a tank to me!"
...and so on. Then when they finally got the picture that we weren't willing to pay to upgrade, they sent us out to the desk in the garage to pick up the keys and we found we'd been allocated the next grade up for free anyway!
Speaking to one of the airport staff when we returnedthe car (who said he used to work for one of the car hire companies), he said that there was so little demand for Economy and Compact cars at MCO (maybe only the Brits want them!), that they only ever have about five of each "in stock" and once they are gone, they have to give everyone a Midsize or above. Of course, they'll still try to get you to pay for this upgrade first...