Diesels produce more power in comparison to any same sized gasoline. That being said, gasoline has better emission characteristics as compared to diesel.

Since trucks essentially are carriage vehicles, their fuel consumption is very high [or the mileage very low], because of the load they carry and their own heavy weight. As such,
you would want to extract as much power from a liter of fuel so as to get better mileage.

Additional information: #-Link-Snipped-#

Emissions are by far the most major concern these days (read a few decades) for the auto industry, and diesels run lean, hence are a source of NOx which is very undesirable and hence highly regulated in all the environmental policies throughout the world. Since petrol engine run near stoichiometry (Equivalence Ratio almost equal to 1), they are lesser polluting as compared to diesels on the NOx front. But when run richer (Equivalence Ratio<1), petrol engines lead to Hydrocarbon emissions, which are again undesirable and highly regulated.

All this being said, there are sequential { DOC (Diesel Oxidation Catalyst, a Catalytic convertor) plus DPF (diesel particulate filter, for trapping particulate matter emissions) } in a diesel engine which have been technologically advanced and can cut out almost 95% of the harmful emissions on a diesel engine. Also with ULSD (Ultra Low Sulfur Diesel), the emissions are reduced further.

I believe, Catalytic Convertors are used in gasoline too. I don't have much info on that.

I am sorry, I presented all this info in a haphazard manner, hope it helps you in some way or other.

As #-Link-Snipped-# rightly pointed out, the post needs clarification on the terms used. So let me make a correction. Diesels do not produce more "peak" power as compared to a same sized gasoline engine.

By "Diesels produce more power in comparison to any same sized gasoline" , I meant the energy content of Diesel is higher per mole as compared to gasoline. As such, at any operating speed, a diesel engine of a certain size will produce more torque than the gasoline counterpart. But, due to the low maximum operating speeds, Diesels dont produce higher peak power as compared to a gasoline engine because:

Power = 2*pi*rpm*Torque

In conclusion, since gasoline engines can operate at higher max rpm as compared to diesels, the peak power rating of a gasoline is always higher than that for a same sized diesel. But for a given rpm (less than the diesel max rpm), a diesel engine produces more power than the gasoline counterpart.