I mean, it is kind of optimal. Fat guys will experience constant, sweaty, skin-on-skin contact in a flight anyway, so placing them together reduces the total constant, sweaty, skin-on-skin contact experienced.
A fat guy next to a skinny person doesn't experience skin-on-skin contact. Which is why I do everything I can to get an aisle seat and hope the middle seat is empty or has a skinny person in it.
Ask the airline, they're the ones that choose narrower seats than the manufacturer's recommendations. I assume this was a budget flight or one of the airlines like American Eagle that service smaller airports?
Narrowing them further can allow even more people to fit, uncomfortably. The fact that you have any space at all is because larger people have to fit somewhere.