Steve Marsden’s

One gram of U-235 can release enough energy during fission to raise the temperature of 66 million gallons of water from 25oC to 100oC! By contrast, to accomplish the same sort of feat by burning pure octane would require 1.65 million gallons of the fuel.

Obtaining such quantities of energy from fission depends on sustaining the splitting of atoms, not a one-time event. The neutrons which are a by-product of the fission process are important in this respect. At least one neutron per fission event must remain within the sample and initiate another fission event in order for the rate of fission to grow. The sample size and geometry needed for this is known as the critical mass

In sub-critical masses the capture ratio for neutrons is less than 1 per fission event and the rate of the process does not grow. Energy output is low. With a critical mass a chain reaction begins which rapidly reaches explosive proportions.