Before nano wireless receivers came out, USB receivers were about the size of a common USB flash drive. They stuck out of the side of a laptop's USB port, begging to be broken off. Nano wireless receivers, on the other hand, are designed to be left in the laptop's port. They are so small that they can rest nearly flush with the side of the laptop. This, according to the manufacturers, lets you pack your laptop in its case without worrying about the receiver damaging the USB port.
If you're a nervous nelly, however, many computer peripheral manufacturers design their mice and keyboards with placeholders for the receiver.