Comparing a 3.5 liter diesel with a 4.8 liter petrol car, that's like comparing apples and oranges.

Topgear on those same cars, better data: "

All the bigger BMW engines are wonderful and the X5 boasts three of the best. You can go for a ‘base' 3.0i petrol(268bhp, 0-62mph in 8.1 and 130mph), the diesel version of the 3.0litre (232bhp, but because of the extra torque 0-62mph in the same 8.1 and the same 130mph top end), or go for the range-topping 4.8-litre V8 with 350bhp, 150mph and 0-62 in 6.5."

So, exactly the same performance for two 3.0 liter engines by a quality manufacturer, one diesel, one petrol.

And yes, if you increase the displacement the petrol engine will get more power but that is no longer a fair comparision.