[Honda100]

Here's a quick comparison, but not exactly what you're looking for. I had a Mitsubishi Legnum (Galant station wagon, 2.4 GDI petrol) that I usually towed 1500 lbs with all over the place. Average mpg, 16.8 US over about 2 years.

I then got a P38 Range Rover, 4.0 V8 petrol, and since it could tow 7700 lbs safely, I was able to load up the trailer to it's max of 3000 lbs (vs. the 1500 I towed with the Mitsu). Average mpg, 15.7 US over a period of 5 months.

The Mitsu had to work really hard around town while the Range Rover was barely breaking a sweat (even with the small engine), so I figure that it's also about efficiencies. If you're straining the small car (remember the Legnum isn't that small), you're going to be getting fairly bad fuel mileage whereas something that isn't straining will return you decent mileage.