I think this was deliberate...IIRC, GM's IDI diesels were built primarily for fuel mileage, not brute power. They were also built lighter (this is why you find them in 1/2 tons and Blazers)...if memory serves, they don't weight that much more than a smallblock g@$$er, whereas other pickup diesels tend to weigh nearly double that of a g@$$er of similar displacement.

I rememebr hearing that a stock 6.2l driven lightly can get 25+ mpg...but it can't tow worth beans (at least compared to the Ford and Dodge diesels) and tends to go through auto trannies like they're going out of style...