I think Patrick has it right. The cast nodular iron is actually a little less dense than real steel, because it has a LOT more carbon, which is lighter than iron.
Here are some numbers:
Ductile or nodular cast iron: 0.257 lb/cu in.
Alloy steel, cast: 0.283 lb/cu in.
1040, 1020 and 1060 steel, wrought: 0.283 lb/cu in.
4340 steel: 0.283 lb/cu in.

THAT MEANS that a cast nodular iron crank with exactly the same volume as a steel crank will weigh about 9% less because the material is less dense.

One word about forging: It results in a better MICROSTRUCTURE than casting the same material, at least in steel. The microstructure is what gives the steel its properties.
R.