First and foremost, one has to know that engines are normally designed to meet set tasks in their day to day lives. It is likely
that a car with a bigger engine weighs more, and is bigger in more ways than one, and so one would expect that “with all things equal” it would have higher fuel consumption.
Thing is, larger engines tend to be harder to turn on because of heavier rotating assembly among other issues which means they use more fuel just ordinarily to maintain speed. Also, larger engines tend to make more horsepower because they can naturally move more air per revolution (all other things being equal). If you take advantage of that additional power by accelerating faster, fuel consumption will suffer again.
In theory
In theory, however, a larger engine could be more efficient than a smaller one and actually use less fuel. For example, an older smaller displacement engine with a cast iron rotating assembly, poor bearing design and oil control, etc. could easily use more fuel in the same application than a newer larger engine with a rotating assembly made of lightweight materials, top grade bearings and oil control, and space age coatings on the contact surfaces of the pistons and cylinders.
With smaller engines, there is always the danger of over capacity. This is where the engine is worked beyond its specified capacity. Over capacity does lead to fuel wastage, higher running cost and is very conducive to a shorter engine life. Larger engines on the other hand, tend to last longer as they almost always operate within their capacity.
When small is not economical at all
If say you drove 800cc, 1400cc and 1800cc engines in otherwise identical cars. All being driven over the same route, by the same drivers, over say a one-day period, you will be surprised to find that the1400cc car uses the least fuel. Many would say that the 800cc car ought to use the least fuel, however, they forget that that 800cc engine does not have the torque of the larger engines to tackle hilly Kampala and overtake swiftly on the highway, so shall often be in a lower gear, leading to increased consumption.
Additionally, other factors like how you drive, maintenance and traffic etc. It is because of this very reason that fuel consumption cannot be proportional to engine size in the real world. It is hard to take all sub factors into consideration, but yes, engine capacity is a factor for increased fuel consumption but the two are not directly proportional.
