If Kirabo and Leandro are standing at the base of a mountain, and Kirabo climbs halfway up the mountain while carrying a balloon, why does the balloon appear more inflated to Leandro watching from the base?

Respond very briefly.
As Kirabo climbs up the mountain, the atmospheric pressure decreases because there is less air above her exerting pressure. The air inside the balloon was initially at the same pressure as at the base of the mountain. When Kirabo moves to an area of lower pressure, the air inside the balloon is now at a higher pressure compared to the surrounding air. Since the balloon is flexible, the higher pressure air inside pushes outward more than it did at the base, causing the balloon to expand. To Leandro, watching from the base, the balloon will appear more inflated due to this increased expansion resulting from the pressure differential between the inside and outside of the balloon.