Air flows from a small bubble to a large bubble when they are connected to each other by a capillary tube.

what is the reason?

The excess pressure inside an air bubble is given by the expression,
P=4T/R
Therefore, the larger bubble will have lesser pressure as compared to smaller bubble. Therefore, air has a tendency to move from a higher pressure to lower pressure and hence, the larger bubble will go on expanding until it bursts or the smaller bubble exhausts.