Helium balloons stay the same size as you hold them, but swell and burst as they rise to high altitudes when you let them go. What are the forces on the inside of the balloon? What are the forces on the outside of the balloon?

Inside balloon outward normal force.

Outside balloon compressing atmospheric force.

Explanation:

At ground level gases outside and helium inside are in static equilibrium with balloon as separating surface. Both balloon and atmospheric forces are compressing to maintain that state.

But as balloon moves upward because of less density of helium gas in balloon internal force of helium gas increases. At high altitude atmospheric pressure decreases due to gravitational pull and fluid nature of gas. So equilibrium reestablish and it start to expand at high altitude.