In the diesel engine, the piston moves to the very top of the cylinder, so the stroke length is longer and as torque equals force multiplied by distance, we have more torque. In order to combust fuel, diesel engines use air compression and with faster compression rate the fuel burns faster thus adding to torque levels.

Why diesel engine has more torque than petrol?

The higher turbo boost pressure, high compression ratio, and greater heat content of the fuel all add to the generation of cylinder pressure that is substantially higher than in gasoline engines too, but it is this continued injection of fuel that really makes the big torque numbers for diesels.

Why are diesel engines more powerful?

Diesels are built to handle turbos. Add more air, and you make more power. The same holds for a gas engine, but they aren't turbocharged as often. Finally, diesel fuel itself packs more energy density than gasoline.