I agree with F3G that it takes all sorts to make a world.
There are many excellent well thought through posts on this thread for such a sensitive subject and whilst I respect the rights of everyone to express their personal opinion, unfortunately the underlying tone of a minority of posts in my opinion, is discriminatory at worse or just downright ignorant at best.
My first flight as a passenger was over 50 years ago and from the mid 60's spent over 35 years of regular air travel mostly on business and for the past 10 years, leisure only.
In that time I can honestly say that I have never been inconvenienced by an obese passenger. Not even one one single occasion.
However, I can categorically state that when travelling economy, I am inconvenienced more often than not by the non-obese passenger sat in the centre of a block of three seats.
The width of an economy seat can be as small as 16". Some are 17" and if you are really lucky maybe 18". Add to this say 2" to the centre of each armrest (1" each side). Now take the dimension across the back from elbow to elbow of an average male when seated (whatever you may consider average) and you will be surprised. Even the most average of average male passenger seated in a centre seat will encroach on the space of adjacent passengers either at elbow or shoulder level.
I have no doubt that your personal space will be invaded if you are unfortunate enough to be seated next to an obese person irrespective of the seat location. However, it is my contention that the chances of your personal space being invaded by an average person seated in a centre seat are much greater.
If a system of refunds were to be introduced for passengers whose personal space was invaded then I feel sure that most passengers would qualify.