Answer: So in an Inflated balloon, the pressure in the inside surface (the pressure by the gas inside the ballon) must be equal to the pressure applied on the outside surface. As the balloon rises, the outside pressure starts to decrease, so the pressure inside must decrease, this allows the surface of the balloon to grow, and the pressures can equilibrate.
Alson you can see it this way, as the outside pressure starts to decrease, there are less force holding the ballon size, then it can get a little bigger.