Some really horrible responses on here.
I don't think the people on the train who moved were necessarily being unreasonable. They may have been uncomfortable, physically or emotionally, and they shouldn't have to put up with discomfort just to spare someone's feelings. Personally, I don't really think of the middle seat as a seat at all, more a barrier between the other two. I wouldn't sit in the middle of two people already occupying the other two and I'm 5ft and a size 4. But that' my personal decision and not necessarily any more 'right' than the OPs (we don't know his dimensions)
But the nasty comments about over weight people in general and the OP in particular are way over the top. Larger people have no choice but take their weight with them. They can't leave it in the luggage rack or put it under their seat.
I hate travelling with my mum (size 26 at least) because I can feel her embarrassment and shame. It's often okay because she can easily share my seat but, in some places, (tube, theatre etc where there are lots of seats in a row) it can be awful. I have seen the appalled looks, eye rolls and sniggers and it makes me feel hurt and sick so goodness knows how she feels. We both have eating disorders but because mine has made me 6 stone and hers has made her (I'm guessing, have no idea) 20 stone then she is somehow the one that's unacceptable and ok to ridicule? It's so uneccessary and unfair. Many fat people shame themselves every day, they don't need other people to do it for them.
It's a bit like the eternal 'wheelchair on a bus' argument - If you find your space 'encroached' on by a large person then it will likely be a less comfortable journey than you anticipated. It might be an awful journey, you might have to move, you might be squashed or feel 'disgusted' etc. But it's one journey. Your next one will probably be fine. The overweight person has to face doing that to person after person every time they make a journey and they can't escape it.