Hello and welcome to Sillyland!
The reasons for the massive torque are thus:
1. The cylinder BMEPs (brake mean effective pressures) are very high;
2. There is no throttle as such;
3. Diesel fuel has a higher calorific value than gasoline litre for litre. (or liter even)
Where I think things might even out is if we compared identical MASSES of diesel and gas. Diesel is considerably denser than gas so we'd have less volume to burn. What I don't want to happen, therefore, is for the oil companies to start selling road fuel by the kilogram!!
In the UK we are now penalised to the tune of about 8p/litre for running diesel cars. So some of the economic advantages have been wiped out.
What a lot of people don't realise (or perhaps, don't WANT to realise) is that modern road car diesels really have progressed enormously.
It is, as PG says, in the fuel management department where this progress has been achieved. Direct injection into the combustion chamber is also helpful with fuel economy. Fuel pressures are insane, as PG has also pointed out. And the fuel delivery is timed very accurately. 3G common rail system can manage up to 4 injection phases per cycle.
Mine, which is quite early, and runs at a 'mere' 1300bar still achieves three phases per cycle. From what I understand the fuel management system delivers an initial dose of fuel to get thing started and then the main injection delivery takes place microseconds afterwards. This keeps combustion noise down to acceptable levels. The third phase is to do with soot filter regeneration, which I'm not going to cover here since it's a topic worthy of its own space.