Meaning/Usage: Used when a person knows someone you know by chance
Explanation: The world is very large with a lot of people in it. When two people know the same person by chance, then the world doesn't seem so big. In this case, the phrase "what a small world" is commonly used.
I didn't know that you knew my uncle. What a small world.
You have family in Florida too? What a small world.
I went to high school with his mom, what a small world.