Does a 6 cylinder 2.8turbo diesel engine consume more fuel than a 2.8 diesel 4 cylinder engine/?

A Jeep Nissan Patrol with a RD28(2.8turbo diesel) engine. Now a 3 door Toyota Land Cruiser Prado (Colorado in the UK) with a 3L (2.8 diesel) motor.The RD28 is six cylinders. The 3L is 4 cylinders. Both displace 2800cc. But different number of cylinders. Take for example both engines and run the motors in a parked vehicle (idle ), which consumes more fuel. My dad's theory is that the 6 cylider powerplant will waste more energy. I say that the number of cylinders is irrelevant because the displacement (cc's) dictates the consumption of fuel.