From Longman Dictionary of Contemporary English(it’s a) small world(it’s a) small worldespecially spoken used to express surprise when you unexpectedly meet someone you know or find out that someone has an unexpected connection to you Did you know David went to school with my brother? It’s a small world, isn’t it? → small
Examples from the Corpus(it’s a) small world• "I graduated from St. John's." "Really? So did my brother. Small world."