to criticize someone who you think is too confident in order to make them feel less confident or less proud.
When he started he thought he knew everything, but we soon cut him down to size.
cut (someone or something) down to size
1. To criticize one in an attempt to deflate their ego. A: "That guy always dominates the class discussion with his rambling, inane thoughts like he's some genius." B: "I know! It's time to cut him down to size."
2. To minimize the importance of something. We need to cut the board down to size before they continue their pattern of overreach.