Don In general terms, how much more expensive is it to run a biggish car
FFS, a 2.0 Mondeo is not "a biggish car" it's half an engine and a repmobile.
Anyway the simple answer is that fuel usage doesn't scale linearly with engine size. This should be fairly obvious - if it did, a 2.0 engined vehicle would use twice as much fuel per mile as a 1.0 engined vehicle and this is fairly obviously untrue.
The drawback with larger engines, larger vehicles is increased weight. The positive side is that the longer the vehicle is, the less drag it has. The final fuel economy is a result of the balance of these factors.
Anyway FWIW, I took a Mondeo 2.2 litre ST diesel up to Manchester and back last year. The average fuel consumption for the return trip was
60mpg, and I wasn't hanging about. OTOH, I've done the same trip in a smaller 1.7 turbo diesel and only achieved 45mpg.You are also missing other important parts of the package, insurance and maintenance costs. Many small vehicles haver terible maintenance costs compared to larger cars simply because the components have to work harder and wear out more quickly and because small cars can be bastards to work on.
Again for comparison, my current big vehicle costs £100-200 for each service. A smaller one I had in the past cost approximately double and every 60,000 miles needed a service costing £1200.