“It’s a small world”
An English expression people often use to

describe when they see someone in an unlikely place.

If I unexpectedly see my teacher at the mall, this is not too surprising. We both live near the mall and go to the mall often.
But if I am travelling in another country, then I see my teacher on the street, I would be surprised. Seems so unlikely my teacher would be in the same place at the same time so far away in another country. Shocking.
To express my surprise of the unlikely meeting, I might approach my teacher and we both might make a comment and say:

"It’s a Small world"—and laugh a little together, enjoying the unlikely moment.
The world can feel a little smaller in situations like these. This is why we say "It’s a small world".