Why does a large piece of pure plutonium-239 spontaneously explode whilst a smaller piece will not

please explain simply :)
thanks in advance

An atom which fissions will create neutrons that go out and run into other atoms that, in turn, will fission, thus creating more neutrons that will run into other atoms, etc. and it will cause a chain reaction. If you don't have enough atoms in one place the neutrons will just escape and not keep the reaction going. But if you keep adding to a ball of plutonium you will eventually reach what they call a critical mass where every neutron will have a good chance of running into another atom of material and then ka-blooey!!!

The decay of a plutonium nucleus produces neutrons which can hit another Pu nucleus and make it fission. Those neutrons in turn hit other Pu nuclei making them fission and causing a chain reaction. For this to work the size of the Pu must be large enough so that the neutrons don't escape before enough of them have been created to cause enough Pu nuclei to undergo fission. Energy is released in each induced fission event because the mass of the reactants is slightly more than the mass of the products and the excess mass is turned into energy by E = mc^2. If the neutrons start to escape before everything can go BANG, then everything just gets very hot, which is the way we like it in a nuclear reactor producing heat energy.