It's a small world.
The world is a small place.
You say 'it's a small world' or 'small world' to express your surprise when you unexpectedly meet someone you know in an unusual place.
... I'm only just recovering from the surprise of running into you like this. Small world.
...My husband Brian and I have been traveling in China when we took a boat trip on the River Li. We were escorted to our place - a table for 8 facing in the direction in which the boat was traveling. When the next two people were escorted to sit opposite us. They said, “Hello Brian and Cheryl” we nearly fell off our seats. They are Meredith and George Park who live here in Waverton. What a small world. Meredith is a Hub member and goes to Strength and Tone classes on Fridays. It is indeed a small world. (wavertonhub.com.au)
You can also use these expressions when you are talking to someone and are surprised to discover that you both know the same person:
I had no idea you knew the Martins. Well, well, it's a small world.