Terrible situation for all concerned, including big person.
On the train it only takes three people to be sat next to each other for everyone to be pressing. It's so unpleasant that many people prefer to stand. It's not very nice for me as it triggers my hidden disability (hidden) symptoms. But of course many people do not like this, disabled or not.
I once sat next to a huge last who did her best not to encroach and who looked ill at ease. There are no winners in these circumstances.