Why Does a Sealed Snack Bag Puff Up at Altitude?

A packet of chips bought at low altitude often looks as though extra air has been added when it is taken up a mountain. No air has appeared from nowhere. What changes is the relationship between the pressure inside and outside the packet. Atmospheric pressure comes from the weight of the air above us, and it generally decreases with altitude. When the packet was sealed lower down, it trapped a fixed amount of gas under those conditions.

A flexible packet can change volume. As outside pressure falls, the internal pressure is initially relatively higher and pushes the film outwards. Expansion reduces the rate at which gas molecules strike each unit area of the packet, lowering the internal pressure. The packet stops growing when the remaining pressure difference is balanced by the tension of the film. In the simplified case of nearly constant temperature and a fixed amount of gas, this is Boyle's law: lower pressure corresponds to greater volume.

The swollen packet is not a precision altimeter. The film has its own tension, temperature affects the gas, and packaging may leak. An aircraft cabin is also pressurised, so its pressure is not the same as the outside atmosphere at flight altitude. The central point is simply that a flexible sealed packet makes an invisible pressure difference visible as a change in volume. Do not heat or puncture swollen packaging merely to test the effect.

https://www.noaa.gov/jetstream/atmosphere/air-pressure
https://openstax.org/books/chemistry-2e/pages/9-2-relating-pressure-volume-amount-and-temperature-the-ideal-gas-law