Actually the difference in power comes due to various processes like turbocharging,supercharging,variable valve timing etc etc.
Hence if two engines are there with same power and torque but different displacement then there may be any one of the above mentioned reasons behind that.
Now for example we have two diesel engines both producing 150 bhp of power and 320 nm of torque while the displacement of one engine is 3000 cc while that of other is 2000 cc,both are using the turbochargers for the power boost.
Now until the turbo starts up,the engine with higher displacement will be better and will be more responsive but as soon as the turbo starts up,then the engine which is lighter in weight will perform better,rest the responsiveness of the higher displacement engine will be still somewhat equal to the lower one.
Its not true that bigger cc engines have less power and torque,but its just the way they are tuned.A Scorpio's MHawk produces less power than a Cruze,but the way the engines behave is different,while the Scorpio's engine is good at delivering quite good torque at any rev range the Cruze suffers from excessive turbo lag.So its just the way engine is tuned,nothing else.
Rest it also matters that at that rev range you are getting the maximum power,like the bigger engine will give all its 150 bhp at about 1000 rpms before the one with 2000 cc.
HENCE YOU CAN SAY..
"THERE IS NO REPLACEMENT TO DISPLACEMENT"-Enzo Ferrari.