Like Storm said, for the same engine size, a Petrol engine can produce more power. But a comparative Diesel engine will generally have more torque. In a 1/4 mile, the petrol will almost always be ahead, but in a roll-on test in one gear, the diesel will usually accelerate quicker.
In terms of out and power though, the most powerful engines in the world are Diesels.
As for reasons, you can say that Diesel engine performance is limited by metallurgy, whereas Petrol engine performance is limited by chemistry of the fuel. It is for this reason that petrol engine power output has not increased to the same extent as in the case of diesel engines over the years. Metallurgy has improved a lot, favouring improvement in diesel engine power development, while fuels have been optimised for polluting less, not making more power.
Cheers!
Sami