They are about the same thing, big=large. Large can be an amount of more than an average size. It can vary on the context in which you use it.

They are synonyms, and can be used interchangeably. There are, however, many idioms involving "large" and "big" that should not be confused. Big Shot - An important person. At Large - Not in confinement, as a whole. "You have a big mouth" - a rude slur said to someone who spreads rumors. "Larger than life" - Very impressive. Don't worry about using big and large in almost every other situation. When you order coffee, for instance, they often use many different adjectives for large: Grande, Vente, Large, Big, Tall. Try learning other magnitudes of large: Gigantic, Titanic, Giant, Humongous, Enormous, Behemoth, Huge, Tremendous, and Gargantuan (my favorite). Johnny

big = informal large = formal