Not part of the USPS small box story, but I ordered a bundle of steel plates that unfortunately fit through our mail slot on our front door, and absolutely destroyed the ceramic tile floor when the delivery person dropped them through that slot.
I, too, felt bad for the guy until I saw him toss it on his shoulder like it was nothing, carried to the garage and gently set it down. Not a big guy, either. :-)
The UPS person who delivered my full size punching bag was not super thrilled. 100 lbs and 150cm tall odd shape. I wish I was there when delivery happened as I was prepared to tip them for the inconvenience.
My dad made a black walnut coffee/end table for me, and instead of waiting for the next time he drove out here, he decided to ship it via FedEx. It was like 100 lbs, but due to the very large size and somewhat wonky shape, he then built a plywood encasement for it with some used carpet to keep the epoxied table from getting scratched. It was quite a sight to behold. The FedEx guy had no problem hauling 100+ lbs of large, awkward object, but he was curious as can be as to what on earth he was delivering.
I ordered a couple 70 pound blocks of tungsten. They were each double boxed and still somehow managed to slip through the first box. Might have been both amusing and annoying to handle those boxes.
My friend worked at UPS. He said you could always tell when someone had bullets shipped because of the rattle and heavy weight. He said those were probably the most common heavy shipment they saw.