it's a difficult one to pin down
there could be many reasons why people have a dislike of or even fear of fat people.

I wouldn't say I have a problem with people such as those mentioned in the OP, but I agree with a lot of the sentiment in the replies and my reasoning is this:

as a teenager and into my 20s I was quite badly overweight, not to the extent of lots of sagging but still classed as obese.

I was never happy with this, never had much interest from the opposite sex, despite being intelligent, amusing and a good listener (all the things that women claim they want but don't, at least when they are young).

I have been losing weight steadily for the past few years through a combination of smoking, a good diet and exercise, but my diet was never particularly bad, and I always did at least some exercise in the past.

My large size was due in part to large portions (partly my fault, partly parents I guess) rather than junk, as well as a large bone structure.

The crux of this rambling spiel is that as someone who was fat and pretty much hated it, I can't stand to see others chowing down on completely unnecessary amounts of food, particularly food that is widely recognised as bad for you (edit: caveat credit to k2k6) especially bad for you in large quantities.
( Vipros. clever got me this far, then tricky got me in, Wed 30 Jul 2008, 14:29, Reply)