It’s a small world
Definition 1 of 1
Pro Tip 1/3Informal Use
The phrase is casual and mostly used in social or informal settings, rather than professional ones. 
Met my neighbor in Paris; it's a small world!
Pro Tip 2/3Positive Surprise
It expresses a pleasant or optimistic feeling of surprise when recognizing connections. 
Guess who I saw at the concert? It's a small world, isn't it?
Pro Tip 3/3Commonality Focus
Highlights the unexpected shared experiences or mutual acquaintances between people. 
Our parents went to the same school? It's a small world!