Porsche 911 mileage question

Road & Track’s Road Test Annual for 1966 has an article on the ‘new’ Porsche 911 that says average fuel consumption was 16 mpg. I seem to recall getting 20-22 mpg in my 911SC, even though its engine was 50% larger than the 1966 model (3 litre vs. 2 litre).

Why did the larger engine burn less fuel than the smaller one?

Is it because the larger one doesn’t work as hard at a given speed (an idea that sounds plausible based on a discussion with a former cow-orker with his 4-cyl. Jeep Wrangler compared to my heavier 6-cyl. Jeep Cherokee, which got better mileage)?

Is it because the 911 had two Solex carburettors, and the 911SC had fuel injection?

Is it simply because engine designs were improved over the years?

Is it a combination of all three, or two of the three?

Also: I’ve noticed that many cars – even American Iron – had surprisingly good mileage figures in the '60s. There are few cars that have been around as long as the 911-series. The Corvette and Mustang come to mind (though both have changed considerably more than the Porsche during their production runs). How have mileage figures changed for other cars that have been in production for a while?