I'll be honest, I think it would depend on the person. If they were friendly enough and polite in so much as saying something like 'let me know if you need me to adjust how I'm sitting a little' then I'd soften up a lot. Likewise, I'd probably ask if they didn't mind if I got up now and again to stretch (and essentially uncramp) if I needed to if they were by the aisle.

I have nothing against the size of another... I don't like confined spaces though... so my irritation wouldn't be against them as such, it would be more down to stress and a sense of sods law... if they were chatty though and friendly that might distract me from the discomfort.

In light of what QiuzzickleII mentioned (and fair play), manners is a two way street... and flying coach is uncomfortable as it is. It is not the fault of either party at the end of the day, airlines try to stuff as many people in as they can and to hell with discomfort... but a bit of consideration between each other goes a long way.