Gear up ...
Within engine itself, type of fuel, type of combustion process (SI, CI or maybe HCCI in future), friction losses, how air-fuel is mixed, etc also do impact fuel economy other than capacity & cylinders. 2 petrol IC engines having 1.2L 4 cylinders won't have the same efficiency necessarily.Please list those other factors and enlighten us. And I have mentioned few others too besides engine capacity.
Outside engine - transmission, aerodynamics, ambient temperature, tires & accessory load (on-board computer, AC, etc) are some other important factors other than weight which impact FE.
Like you can't get an idea of the performance of a CPU by looking just at its number of cores or image quality looking at MP of a camera sensor, similarly FE of a vehicle depends on a lot of factors.