Also, the compression ratio of a diesel engine is much higer then a petrol engine, so the air/fuel mix is more compressed.

Higher Compression = More Power/Torque

Demonstrated by all you turbo boys whacking up the boost and N/A boys skimming heads but tones.

But also as it produces so much more torque at a low rev range it can keep the engine turning over where as a petrol engine produces less torque so needs to rev higher to keep the momentum of the pistons and crank up.