That's what our carrier does as well, but I think Ed was referring to one of those "community/central" mailboxes shared by a dozen or so families.
The one's I've seen have a couple of large (as it 2 - 4 x's the size of the individual boxes) package bins on them. When the carrier has a package that won't fit in the individual mailbox, the place it in one of the bins and place the key for the bin in the mailbox.
This is similar to what they do in the post office itself when an item arrives that won't fit in the smaller PO Box.
What they do for the occasional item too big to fit in the package bin is anyone's guess. I suppose they could fall back to the common sense approach and stop by the individual house, but that would make sense and we ARE dealing with a government created agency and a union so...