I have had such an event occur and the same airline and flight attendants that refused to let me fly with one leg in a cast and the arm rest of the empty adjacent seat raised "for safety reasons" -- permitted the obese person to pull up the arm rest before being seated, required a seat belt extender, then proceeded to place their drink on my table top, as my "seat mate" was unable to lower hers. I believe that happened because I happen to be 5' 1" and weigh 104 lbs so they assumed I did NOT NEED a whole seat to myself.
There will be no next time, for that ever again to happen!!! I'll demand a seat in first class, a different seat for the person using my seat as well as his/hers, or he/she purchase two seats, as long as they require that much room!
I never had my seatbelt on the whole flight - I think it got stuck in one of the 'fat folds' of the lady next to me. Makes no difference, they both had me pinned in to where I couldn't move - One of them sat on my magazine and that was the last I saw off it.
On the flight they gave a snack which included a muffin - Having just come off an international flight I wasn't hungry and the lady on my left asked if I was going to eat my muffin - I said 'no' and then passed it to her and the other lady seemed to give me a weird look - kinda like I should have asked her as well -- to which I replied "Did you want me to cut it in half?" -- I felt really weirded out and wondered if I should ask the stewardess for another muffin to feed the other one - It was horrible - they both slept afterwards and made weird sounds.
I'll never forget that flight for the rest of my life.
This sounds very much like ...to each according to his needs and from each according to his abilities....scary stuff.
My experience on a cross country flight next to an overflowing woman included very strong perfume.