I can't really see that as being the major driving force. I would guess that the majority of the wire in a car's loom is 18ga or smaller anyway. As the number of sensors, controllers, and 'low' power extras has gone up in recent years, I'd bet that the average wire diameter has decreased, and as such the potential % savings aren't huge.
But what might be a factor is EMI and SnR ratios. Electromagnetic radiation is proportional to current, so by cutting the large currents by 3 will reduce potential EMI problems. EMI issues are becoming more prevalent - in modern cars EMI isn't just something that makes your car stereo click and whine, but something that has the potential to prevent safety-critical components (brakes, airbags etc) from working correctly.