Economy Car Hire Ltd. Have I Been Treated Fairly?

Kenman

I'm trying to work out if I received what I'd paid for when I booked a car with Economy Car Hire (based in Dereham, Norfolk), for my trip to Boston last month.
I wanted a fairly large car, so booked a "Ford Fusion or similar" which their voucher for an Alamo car also described as "Group D2 SDAR" although I've no idea what this means.
When we went to pick up the car, the Alamo man said our voucher wouldn't qualify for a Ford Fusion or similar, but was for a Chrysler 300 or similar. He wouldn't budge.
When we went up to the garage, the car we were given was a VW Jetta, which was comfortable enough, but a lot smaller than the car we were expecting.
I sent an email complaint to Economy Car Hire as soon as we arrived at our destination 80 miles from Boston and, after a bit of pressure, they suggested I either bring the car back to Boston or take it to the next nearest Alamo depot in Hartford, over 100 miles away. I had to tell them that I wasn't going to spend any more holiday time trying to change the car for something bigger (which they didn't actually offer) and that we'd have to put up with the Jetta.
On my return, I again complained to Economy Car Hire, but they've told me "you were actually provided with an upgrade as the VW Jetta is in a higher group compared with the Ford Fusion". Given that it's significantly smaller, I'm not convinced.
Any thoughts?

NiftyDigits

Wheelbase 2,650 mm (104.3 in)
Hybrid: 2,655 mm (104.5 in)
Length 4,640 mm (182.7 in)
Hybrid: 4,628 mm (182.2 in)
Width 1,780 mm (70.1 in)
Hybrid: 1,778 mm (70.0 in)
Height 1,450 mm (57.1 in)
Hybrid: 1,453 mm (57.2 in)

Wheelbase 107.4 in (2,728 mm)
Length 190.6 in (4,841 mm)
Width 72.2 in (1,834 mm)
Height 56.9 in (1,445 mm)

Depends on the age of the car. They might argue same class...but it definitely is not an upgrade.