If you look at the Cummins website, all of the various versions of ISB's have considerably lower horsepower output than the version that Dodge uses in their pickups. Bigger demand applications with higher duty cycles require more moderate power output from the engine to ensure longevity of service.
Really though, no pickup diesel engine needs to produce 400 hp and 800 lb-ft torque. It's just rediculus what theyre tuning them for now days. It's no surprise the fuel mileage is no where near what it should be. Take that same engine and tune it for 250 hp and 550 lb-ft and it would use less fuel and still operate just fine for use in a pickup.