RFID sizes are more about the coil which must be used to not only pick up energy for power but also modulate the field to transmit data back to the transceiver.

RFID card, the coil typically goes around the whole outside of the card. Pet ID chips (which is probably what Gates is talking about) have a very small coil but the range is extremely limited (which is why they are typically placed in a standard place on a pet).