The Egyptians made pyramids with a heavy bottom and light top. An inverted pyramid would have been so unstable that a mere gust of wind could have destroyed it. Something similar happens in pouch bags where heavy products stay near the bottom and lighter air fills the upper parts of a pouch. The center of gravity goes down to even lower parts as the product is consumed. Therefore stability increases as consumers empty a pouch. An empty pouch, however, is not very stable because of its negligible weight.
When it comes to stability, pouches are even more stable than long bottles and jars. The underlying reason is once again a lower center of gravity in stand up pouches.