I have been told, by (if I'm remembering my source right) someone who has worked as a UPS driver for 30+ years, that Amazon does that on purpose. Because the variable they're optimizing for is not "are we wasting cardboard". Cardboard is a renewable resource, which also recycles really well, so a little wasted cardboard is not a big deal. What they're optimizing for is packing the truck. Your item arrived in a too-big box, because that box (and the air within it) was calculated to fit exactly into what would otherwise have been empty space in the truck if the box had been smaller. In other words, you know how sometimes inside a carboard box, you'll find the item you ordered plus another smaller (and empty) cardboard box used for filler space? That's exactly what they were doing, with your box as the filler space in the truck so that other boxes wouldn't slide around and damage their contents.

... 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.