Bealzebub,
A very reasonable and middle-course reply, but................
What would you do in a situation where I, a passenger of normal size, find that my seat, which happens to be the last one on the aircraft, is partly occupied by a fattie. I, unable to face the prospect of, say 2 hours+ of being squashed by this sweaty and smelly mound of flesh (as it will invaribaly be), tell the CC clamly and politely that I will not accept that seat which, for all practical purposes, is unavailable.
Are you going to offload me or the fattie? It's decision time, and an exploration of the issues, let's use our common sense and so forth, won't cut the mustard. Me or the fattie; which is it to be? Someone has got to get off, and if it's me I'll sue.
(Let's assume that no other passenger agrees to take my seat, and that the seat really is unusable while the fattie is there, wherever the armrest is.)