How does a mass transportation system help in saving energy?

For example, a bus can carry, say, 20 people (or more, depending on its size), while a personal car may carry 4 people, but often carries much less. Let's assume it carries 2 people on average. Now, the bus, being bigger and heavier, will certainly require more fuel - but it won't need 10 times more fuel, but less than that. Thus, fuel (energy) will be saved if people use the bus instead of a private car.