This feels like a silly way to do things. Why are they storing arbitrary cuboid items in fixed-size boxes? If they designed the warehouse for maximum robot efficiency, doesn't it seem more sensible, on first glance, to have stacks of the same item in a regular grid accessed from the top, instead of optimizing how to pack wildly different items in the same box?
From working on warehouse picking software way before there were robots ... it's because humans. It would take longer for a human to find space in the right space -- or instead, be lazy and forcibly put an item in a space it barely fits -- than to simply dump it into an arbitrary box with enough space and record the box. Then when deciding the path for the picker to pick items from, there's a high chance one of the purchased items is actually nearby other purchased items.
The Kiva warehouses deliver the pods to human pickers at fixed stations. The humans just have to retrieve the product from the indicated bin without roaming the warehouse. The storage area for the pods is kept maximally packed without any aisles since the robots have their grid to operate in underneath.
