LOL! You guys crack me up!
But, yes, it is not uncommon to see one fish that is essentially a porker in comparison to the others. You can try to feed the other fish in the direction that this little piggy is not, but you will see that he will quickly swim over to gorge himself.
Koi are just greedy eaters by nature, so you might just allow for this one fish to enjoy his life and accept the fact that he's going to be the "BIG" man in the pond. As long as you've ruled out any illness, and you are certain that it's just a weight issue--you don't have to fret. Take note, that as with humans, that this can likely shorten the life expectancy of this particular fish--but I would just let the poor chubby beast enjoy himself.
With winter coming, he'll likely lose most of that tubbiness come spring anyway.
Unless, of course, you are ready to quarantine him and put him on a diet?