I will have to disagree a little. It depends on how well that diesel engines burns said fuel. If you are putting 100 hp worth of fuel in the engine that is blowing black smoke due to insufficient amount of air then you will be using more fuel than a diesel that is has plenty of air to burn the fuel which means it would need less fuel to make 100 hp. Today's turbos that spool faster wastes less fuel since the engine is getting enough air to burn the fuel off the line meaning you would need less fuel to do the same work.
There is also the factor of fuel pressures and injection events. With higher fuel pressure means the fuel is being atomized better which makes it burn more completely meaning you will get more out of 1 gallon of fuel versus and engine is not burning it as completely. Also, with having multiple injection events per stroke with today's diesels, you can injected fuel in spurts as the piston travels down during the power stroke making more torque and a complete burn instead of all at once meaning you will not burn all the fuel being injected. This means you would need less fuel to make the same torque since you are getting more out of the fuel injected.
Case in point is today's small diesels like the 3.0L Ecodiesel making the same amount of power(240 hp 420 lb-ft) as an old 5.9L did back when it first came out and yet uses a lot less fuel. Fuel economy is all about getting the more energy out of a single gallon of fuel which means you need less of it to do the same work.
