[cosmicray]

Volume isn't really the issue, but mass might be. In the case of the toner cartridge (which appears to have been a sub-assembly of an actual printer) a truly devious individual might have removed the correct amount of toner, and then offset it with an equal amount of explosive material. The mass doesn't change (mostly due to the fact that the toner cartridge had excess space in it to begin with).

What does change is the chemical signature of the contents. That would require inspection of each item as it is presented for shipment. Hence the problem.