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. 
