The fission process releases neutrons, so you effectively "add neutrons" by concentrating self-fissioning material in a small space. To put another way: If you

The fission process releases neutrons, so you effectively "add neutrons" by concentrating self-fissioning material in a small space. To put another way: If you increase the size of a sphere of fissionable material, its rate of "natural" decay stays the same, but more of these decays lead to neutrons triggering additional fission reactions. So doubling the size of the sphere leads to >>2x fission events due to neutrons having more fissile material to hit on the way out. A large fission reactor is just a scaled up version of this with control rods to adsorb extra neutrons to control secondary "induced" fission.