Reducing weight and improving fuel-economyOriginally, the Chevrolet Volt's gasoline range-extender was supposed to be a 1-liter, 3-cylinder engine. That didn't happen, and the current production model has a 4-cylinder with 1.4-liter of displacement. But apparently, GM engineers haven't given up and the next revision of the Volt (likely in 2015) could have a 3-cylinder engine to help reduce weight and further improve fuel-economy. The same thing could happen to the new Cadillac ELR in 2016, also making it lighter and more efficient.
None of this is official yet, and car-makers don't like to talk too much about their strategy years in advance, but it would make sense. Over time plug-in hybrids like the Volt and ELR should be getting better batteries that provide longer all-electric driving range, so the gas engine becomes more of a dead weight to carry around. Downsizing it extends the electric driving range, but you still have a backup when you need it. By using cutting edge technologies, it's possible to squeeze a lot of power out of a small displacement engine, so it's possible that not much power would be lost; it could be pure gain!
This would be part of a broader effort at GM to improve fuel economy and cut weight. In a recent speech, GM CEO Dan Akerson set a target of 15% in weight reduction. He also recently mentioning work being done on an electric car with 200 miles of range.