Edit to comply with rules: it’s to do with the tension on the surface of the bubble vs the tension on the surface of the water.
This is a quite accessible explanation from StackExchange: [https://physics.stackexchange.com/questions/108340/what-is-the-effect-of-surface-tension-on-an-air-bubble-inside-water](https://physics.stackexchange.com/questions/108340/what-is-the-effect-of-surface-tension-on-an-air-bubble-inside-water)
Latest Answers