Large capacity engines tend to be thirsty. They are also terribly inefficient at low speed, as the throttle is nearly closed so the additional cylinders are running at very low pressure, resulting in an engine working hard to draw air so it can feed fuel into all the cylinders.
Many manufacturers have now adopted technologies to turn off the additional cylinders under low power conditions. Less cylinders drawing air from the intake manifold mean less fuel needed as we are effectively feeding a smaller engine.
The cylinders are deactivated by simply keeping the intake and exhaust valves closed, trapping the exhaust gases from the previous cycle in the cylinder, thereby reducing the need for any air or fuel.