I'm going to make the person in the window seat put down their arm rest even though the passenger is too obese to do so
Why would you want them to be so uncomfortable?
They put down it down to eat, disrupting the people at the middle and other seats.
I can see how that would be frustrating.   Keep in mind that this is something that cannot be helped due to their size, you are just creating a tense situation for everyone by trying to make them do it.
That person just ate a can of potato chips and he couldn't put it down before.
Exactly, I think that it seems a little mean to ask him to keep it down. You should not try to make people do things that they are physically not capable of doing.