affected by the big fatties who spill into the seats next to them, but I also think buying two seats isn’t entirely fair to said fatties either.

Could there be some middle ground of having a couple “XL rows” in the back of the plane that have more space by having one less seat than other rows? Maybe it costs a tad extra and is exclusively for large flyers (who have to register as such with the airline maybe), but it’s not the cost of buying two seats. If no XL flyers buy the seats or get offered at the gate to take those seats, they then get used for standby passengers.