Any system capable of fusion is going to yield more energy from turning into Iron then turning into anything else.
If you fused into something bigger, it would still be more efficient to split that atom and fuse it's nucleons into more Iron.
Atoms are mostly empty space - chemical lattices are mostly empty space - so in something like a star you can make super-dense iron and turn anything else kicking around into more iron.
EDIT: I feel like the problem here if you're assuming that the size of an atomic nucleus is anything other then negligible compared to it's electron shells. There's no situation under which a non-neutron star will "fit" more of a heavier nucleus in, compared to just densifying iron.
>If you fused into something bigger, it would still be more efficient to split that atom and fuse it's nucleons into more Iron.
Of course, but that doesn't easily happen, as the nucleons are already bound together into heavier nuclides.
In fact I guess that might happen in supernovas: The main sequence burns into lead, which is then reconstituted into the iron-nickel mixture that we know from iron meteorites, which also releases energy due to the higher average binding energy per nucleon, but needs extreme conditions to occur.
Except in a star anytime you get two particles interacting enough to potentially fuse, that's potentially a fission event as well. There's no rule which says any given nuclear interaction is fission or fusion: that's what we conclude after it happens.
So in a star creating regular fusion events, even an errant heavier nuclei is going to eventually be decayed down to Fe (or head off to become a heavy element in the upper crust).
That just doesn't happen. We know that elements are stable. It doesn't normally happen that e.g. gold spontaneously explodes into iron, you'd need the extreme conditions inside a supernova to make that happen.
If you fused into something bigger, it would still be more efficient to split that atom and fuse it's nucleons into more Iron.
Atoms are mostly empty space - chemical lattices are mostly empty space - so in something like a star you can make super-dense iron and turn anything else kicking around into more iron.
EDIT: I feel like the problem here if you're assuming that the size of an atomic nucleus is anything other then negligible compared to it's electron shells. There's no situation under which a non-neutron star will "fit" more of a heavier nucleus in, compared to just densifying iron.