Fusion of tritium and deuterium
    Input:
      - 1 deuterium atom: 1 proton and 1 neutron
      - 1 tritium atom: 1 proton and 2 neutrons
    Output:
      - 1 helium atom: 2 protons and 2 neutrons
    Overall: 5 particles become 4, 20% become free to hit other atoms or turn into energy.

  Decay of uranium-235 into barium and krypton
    Input:
      - 1 atom of uranium: 235 particles
    Output:
      - 1 atom of barium: 144 particles
      - 1 atom of krypton: 89 particles
    Overall: 235 particles become 233, less than 1% become free to hit other atoms or turn into energy.

Fission is much more similar to radioactive decay: big atoms turn into smaller atoms releasing a few free particles. Fusion releases basically the same amount of particles with much smaller atoms.

Of course, this is an extreme simplification of the process. Energy required to start each reaction is ignored, but the idea is mostly correct, I think.

EDIT: fixed tritium number of protons and neutrons and grammar.