Why does fusion generate such large amounts of energy. If fission releases energy when some mass is lost as energy, then shouldn't the fusion process absorb energy to fuse nuclei together?

Why does fusion generate such large amounts of energy. If fission releases energy when some mass is lost as energy, then shouldn't the fusion process absorb energy to fuse nuclei together?
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Abraham Norris
Fission is exothermic only for heavy elements, while fusion is exothermic only for light elements. Intermediate nuclei, in the iron/nickel range, are the most tightly bound, and so you generally release energy moving in that direction.
Fusing stable elements into uranium would consume energy, as would trying to break helium into hydrogen.
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Emmanuel Pace
The strong force is attractive at short range and wins over the electromagnetic repulsion between protons. Pull the protons apart a little and you will get fission because the electromagnetic force wins. Conversely, push protons together and you will eventually get fusion when the strong force takes over from electromagnetic. For large nuclei the electromagnetic energy required for fusion is larger than the energy returned by the strong force - so you will only get net energy out from fission. For small nuclei it's the other way round - the strong force releases more energy than the electromagnetic force takes to trigger fusion. In this case you will only get net enerrgy out from fusion.