The primary is generally built as small as possible, due to the fact that the energy released by the secondary is much larger, and thus building a larger primary is generally inefficient. There is a lower limit on the size of the primary that can be built, known as the critical mass. For weapons grade plutonium, this is around 10 kg. This can be reduced through the use of neutron reflectors or clever arrangements of explosives to compress the core, but these methods generally add to the size and complexity of the resulting device.