When the purpose of the diesel is to move air ( with fuel added of course ) I'd say size does matter.
Look at all the problems Ford had when they tried to make more power with less displacement.
Cummins has been in pickups how long? Since 1990?
When hasen't Ford had engine problems? Size is part of it, but theres a lot more to it for HP and torque, there is a reason why the 12v cummins made so much torque for the power and the newer diesels since still cannot match it hp/tq ratio wise. There is so much more to a diesel then it's size. Look at the change from the Cummins, 5.9L for almost 20 years and the power doubled.
I see how long they've been around as a moot point. They have the best engine for pickups hands down.