Hello, this expression exist in Arabic. There are many ways to say this one. First of all, I do say something; about the expression of Misaki or Wael are totally correct to say in Arabic (MSA).
Well, I will try to collect every natural expression in Arabic. In this sentence, we equivalent world rarely by al-3aalam, we do use: a-ddunya.
1 آه! كم الدنيا صغيرة
2 الدنيا صغيرة
3 كم الدنيا صغيرة
4 صحيح، الدنيا صغيرة
5 واللهِ لَلدنيا صغيرة