Due to greater compression ratios and higher burning temperatures the diesel engine has a thermodynamic cycle that is basically more efficient than that of the gasoline engine. Then it uses less fuel per unit of power produced, but in order to do this it needs more space inside the cylinders to make the best use of the more extreme pressure and gas expansion characteristic.
Consequently, when the two engines being compared have the same volumetric capacities, than the amount of power of the gasoline engine is more, but it takes more fuel and a greater heat loss to achieve this.
Whilst Diesel engines may have less power, they have much more torque than petrol engines. This means they perform as well in terms of acceleration and speed irrespective of how full the car is, or if its going up a hill etc. In petrol engines there is a much more noticeable drop in performance when heavy laden or going up hills.
