Whilst I think its deplorable to label customers based on their physical attributes, I nevertheless disagree that calling someone fat should be an insult. It is not inherited or inflicted (apart from a few rare conditions), it is a choice. We do not have to have 9 million people a year die from hypertension while 4.5 million die from starvation. Everyone’s decisions affect each other, just because we physically remove ourselves from the consequences of our actions it doesn’t mean there aren’t any. Furthermore, a Body Mass Index of ~18-25 is not a desire, it’s a biological necessity for healthy body function. You don’t get to demand the world stop telling you the truth that would save your life just to spare you some embarrassment.
In terms of self image – if you find it insulting that somebody calls you fat, either accept that personal image is not as important as character (obviously) or do something about it.
[This is obviously not to say that it's morally acceptable to walk up to someone and call them 'fatty fatty fat fat'. It is the intent behind the words that should be examined, not the words themselves. Anything can be turned into an insult if it's said the right way.]