Big" and "Large" mean the same thing. It's a big crocodile. It's a large crocodile. Large is more formal than "Big" you say a farge posterior and a big bottom horses have large posterior. " Big" is used to refer to old siblings. He's playing with his big brother. And "big" can mean importance or major. Moving to another country is a big decsion. "Big" is more common and is used in a number of fixed expressions Large felines such as lions and tiger are called big cat.