Why might the olmec have carved their leaders' heads in colossal size?

They wanted to present them as if they were gods. This was quite common in history and is a reason for example why pharaohs got pyramids built in their honor. The giant heads depicted their leaders as gods or someone who is in general larger than life itself. It was a sign of respect and others who would visit would also see this and know who they are dealing with. Such heads can still be found and examined which is important when trying to understand their ancient culture.