Bob Lutz Seems to Think So...Bob Lutz, who, when he was still at GM, spearheaded the herculean task of pushing the Chevrolet Volt project through GM's bureaucracy (the company seems more nimble in its post-bankruptcy days, but back then it was something else entirely) seems to have some regrets about the choices that were made back then. Indeed, the father of the Volt told Autocar that he now believes that the Volt's technology is wasted on a small car and should instead go into much bigger vehicles.
That's an interesting way to look at it, and Mr. Lutz makes a good point. While we tend to think about fuel savings in relative terms ("this vehicle is twice as fuel efficient as this one"), all that our planet cars about is absolute numbers. How many gigatons of carbon dioxide go in the atmosphere each year? How many smog-forming pollutants are released? Etc.
So it's true that bigger savings can be had by improving the least fuel-efficient vehicles first with all else being equal. This strategy also has the advantage that bigger and less fuel efficient vehicles tend to be more expensive, and so their buyers are more ready to spend the cash on these new technologies at the time when they are most expensive (over time they get less expensive thanks to technological progress and economies of scale).
This makes sense as long as all else stays the same, but I don't think it should. It's a great idea to have much more efficient big trucks and SUVs (ideally running 100% on electricity from clean sources), but that should be for those who need them. Over the past few decades, cheap oil prices and no pricing being put on negative environmental impacts (aka externalities) has pushed people to go for bloated vehicles they don't think. A contractor might need a huge pickup truck, but there are many people who have them but barely ever carry a few bags of groceries. So we should improve all kinds of vehicles - small, big - but also encourage people to realistically assess their needs when they pick a vehicle, and also to consider alternatives like walking, biking, car-sharing, living closer to work, etc.