How close each nuclei is to each other matters. The neutrons get emitted randomly and if everything is packed together tightly that doesn't matter because no matter where they go they're going to hit another fissile nucleus. Once the reaction begins to emit energy it's going to push that core apart and now some of the nuclei being emitted are going to miss a fissile nucleus and just go away into the surrounding environment. Eventually you also convert enough fissile nuclei to non-fissile and they might just absorb the neutron taking it out of the reaction without a new fission. But that won't matter for a bomb, it becomes more of an issue in reactors.