There are a few difference between the material used between carbon steel and cast iron pans. When it comes to composition, don’t let the names fool you. Carbon steel is designed with 99 percent iron and only one percent carbon—while cast iron is actually made up of three percent carbon!
Because of all that extra carbon, cast iron pans have more angular sides while those from carbon steel are rounded out. In general, carbon steel pans are also much thinner than cast iron—and much lighter in weight.
For a short answer: the shape and weight of the pan make all the difference.