As incredible as it sounds, in the late 1700s, men and women took baths only twice a year--May and October! Women kept their hair covered, while men shaved their heads (because of lice and bugs) and wore wigs. Wealthy men could afford good wigs made from wool. They couldn't wash the wigs, so to clean them they would carve out a loaf of bread, put the wig in the shell, and bake it for 30 minutes. The heat would make the wig big and fluffy, hence the term "big wig."
Today we often use the term "here comes the Big Wig" because someone appears to be powerful and wealthy.