Curley has "done quite a bit in" boxing. He has done a great deal of boxing. This is actually important in the book.
Curley's ability to box sort of feeds in to his personality. He is a very arrogant and aggressive person. He likes to pick on people with a combination of his boxing ability and the fact that his father owns the ranch. These allow him not to have to worry about people beating him up.
This arrogance gets him in trouble when he goes after Lennie and gets his hand crushed.