[queensnake]

How about that - I thought it was all about the binding energy curve: http://ec.europa.eu/research/energy/fi/fi_bs/images/fig2_147...

where, as the diagram shows, fission comes up from the heavier elements (less energy difference) and fusion from the lighter (greatly more).

[neutronicus]

Per fusion event, you get about 14 MeV to the about 200 MeV you get per fission event. It's true that the binding energy per nucleon is higher, but there are many fewer nucleons.