Bronze is only slightly heavier than iron (~10%) but bronze was more supple and less brittle than early iron weapons. To make them useful early iron weapons had to be heavier than an equivalent bronze weapon. The advantage of iron weapons was that iron was cheap and plentiful while bronze was expensive. When people figured out how to get a fire hot enough to add sufficient carbon to iron to make it as supple as bronze (early steel). The lighter steel weapon had to be swung harder to do the same amount of damage that the equivalent bronze weapon did.

The advantage of the steel weapon over the bronze weapon (but not the iron weapon) was that the steel weapon held a sharper edge and held it for longer. The bronze weapon's advantage was that it did not corrode or rust like iron and steel weapons.