It's about robustness, longevity and power delivery characteristics (and many other factors besides!)
BMW make a 2l turbo-diesel that produces more power and torque (on a test bed) than a 4.2l TD in an 80. But the 4.2 is MUCH less stressed - brake mean effective pressure is lower, compression ratio is lower, operating rpm is generally lower and so on. Your big 4x4 is typically used for towing, slogging around farms, overlanding (more so than a 2l BMW!) which are more demanding on an engine and the inconvenience of engine problems is much more significant off the beaten track than at the side of a tar road. Slow speeds mean cooling is critical, dusty environments mean accelerated engine wear. Offroad use often requires very low engine speeds, good engine braking and plenty of torque available from idle. All characteristics provided by bigger engines generally.
The 4x4 also does more "hours" for the same mileage than a roadcar, which could average 40-60mph over its life. So 200k miles offroad will mean probably double the engine hours compared with 200k miles in your BMW 2l commuter. So you need a big tough engine if you want the truck to last.
Farmers, construction companies and weekend offroad warriors expect their 4x4s to last a decent length of time. So Mr T and his close friends at Nissan stuck in big, unstressed diesel engines in their heavy duty trucks and have earned a great reputation as a result.
I am interested to see how the new VW Amarok pick-up with its high speed (for a diesel) 2l TDi engine stacks up against the Hiluxs etc over time. This is obviously a different concept (lightweight, good fuel economy) to the traditional heavy-duty, lower performance route most truck manufacturers have opted for.
EDIT: As a post script, remember that engines are simply a tool for converting chemical energy in fuel into kinetic energy. In theory if you burn a given quantity of fuel, you generate a certain quantity of energy. Of course in practice we know that there are many inefficiencies, which make some engines produce more or less power than others. The principle still holds - if you can burn as much fuel in a 2l engine as 4l engine, you can produce similar power. The challenge for an engine is whether it can cope with that power for its entire design life in a variety of changing and testing conditions.
Cheers,