Answer : Both fission and fusion create far more energy out of far less matter than combustion.
Explanation : Nuclear fusion reaction involves fusing of two atomic nuclei of elements combines to form one or more different atomic nuclei and subatomic particles (namely neutrons or protons). Whereas the nuclear fission, is exactly opposite of fusion reactions; in which one big atomic nucleus of an element splits into smaller parts usually creating many lighter nuclei.
So, it is clear from the above statements that nuclear fusion and fission reactions far more energy out less amount of matter. As it uses only the atomic nucleus of the elements to generate nuclear energy from it.
