( saying ) used to express your surprise when you meet sb you know in an unexpected place, or when you are talking to sb and find out that you both know the same person
IT'S A SMALL WORLD
Meaning of IT'S A SMALL WORLD in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005