How about if Large Person A books two seats but Large Person B doesn't. Large Person A's seats are split and Thin Person C (who is seated next to Large Person A) wants to move, but the only seat left is Large Person A's second seat which is next to Large Person B *and* is in an exit row. Large Person A does not speak English.

On a more serious note: Any larger people who book 2 seats should really get the miles for both seats *and* a bonus! They doing themselves, the airline and their fellow pax a favor!