In a lot of cases it's optimization for a truck loadout. The loading setup is usually automated to maximize the number of packages in a truck while minimizing

In a lot of cases it's optimization for a truck loadout. The loading setup is usually automated to maximize the number of packages in a truck while minimizing empty space, so you can easily end up needing a larger box to fit a specific gap in the truck. There are also some products that are marked as "fragile" and require a minimum amount of empty space in a box to prevent them from getting smashed. The exact definition of "fragile" and amount of space required is some kind of arcane black magic.