... I see someone else has posted this elsewhere in the comment thread. Eh, I might as well post this anyway, because it's confirmation from a different source.
Except: The hypothetical perfectly-packed 53' trailer that leaves the originating warehouse is not the same trailer that delivers stuff to my doorstep. Things get sorted and re-sorted as they move along. It ultimately becomes random instead of optimized, and these random giant boxes take up a lot of space in local delivery vehicles.
Besides, the exceptions can be too exceptional to support any notion of it being deliberate.
It's difficult to describe the biggest box I've ever gotten from Amazon, except to say that it was too big to fit onto the seat of the recliner by the door where I usually put these things. I've received full-size, assembled, 1990s tower PCs in smaller boxes.
Inside of that exceptional box was just 3 ethernet cables, each 1 foot long, that cost me less than $1 each. That whole box could have been a brown paper envelope.
Plus, I seem to recall that they also optimize by giving the driver a route to follow and planning the boxes to be packed in order, so that only one row is being emptied at a time. I know my UPS driver friend has told me UPS does this, and it's an obvious optimization so I'm sure Amazon does it too.