Not a physicist, but as I recall it's because the relevant curve describing energy states doesn't slope just one direction. It goes up and then down again: htt

Not a physicist, but as I recall it's because the relevant curve describing energy states doesn't slope just one direction. It goes up and then down again: http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin.html#c2 http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin.htm... This means when talking about tiny nuclei (hydrogen and helium), moving to a bigger nucleus (fusion) takes you to lower energy (more stable) state and releases energy. But when you're talking huge nuclei, it's the opposite, and moving to a smaller nucleus takes you to a lower energy state and releases energy.