Your answer indicates you lack the most fundamental understanding about how nuclear fission works. There are different kinds of fission. Spontaneous fission takes place only in the largest nuclei such as those with an atomic number of 232 or more. Then there is fission from fast, intermediate, slow and delayed neutrons. Fast fission is also called prompt fission.
Prompt criticality is very undesirable in a nuclear power plant. When it happens there is a steam explosion if it is a water moderated reactor or a fire and explosion if it is sodium cooled. But in neither case is the explosion exactly like that of a nuclear bomb. In a nuclear power plant, fast intermediate, slow and delayed neutrons are all taking part in supplying the number of neutrons required to stay critical or slightly super-critical. Read abut the US Army’s SL-1 disaster.
In a bomb, the most desirable condition after assembly of the critical mass is prompt criticality before the slow neutrons can cause more fission that change the shape of the physics package and make the bomb fizzle. In this case assembly of the critical mass means to bring the pieces of the U-235 or Pu-239 together by high explosives quickly and compress them.
If there is no critical mass, there is no prompt criticality. Prompt criticality relies on enough fissionable material in close proximity so that the release of the 2-3 neutrons per fission can each result in more than one fission. This rapidly escalating effect is what causes the nuclear explosion. With more material in excess of critical mass, you can get a bigger blast, but if you are under critical mass, then you get no nuclear yield at all, just a small amount of radioactivity from the original uranium or plutonium scattered about by the high explosives in the bomb.
This is why it is called critical mass and not okie-odie mass.
It is very critical to the operation, without that amount available, no atomic boom boom.
Ranb