I have two points:
1. If "fat" is derogatory, while "thin" and "tall" are not, then these are value judgments made throughout our contemporary culture. "Rubenesque" is another way of saying fat, and it would have been acceptable years ago. Nowadays it's a snide euphemism. If you want to be "fat", you have every freedom to do so; as a nation we proudly defend and celebrate your right to do so; but you cannot then complain if somebody calls you that as a mere differentiator, and not as a moral judgment. If you don't like being called fat, don't be fat. Yes, it IS that simple. If it is your choice, then you have not right to fault me for observing your choice. Why should I have to support you in your denial? You can't have it both ways. If it is not your choice, then it is a mere statement of relative size, and society has no right to judge people negatively on that account.
I DO realize that the above sounds insensitive. I am not making a value judgment on what is , merely commenting on the state of our culture, and its propensity for emotional value, not consistent logic.
2. Given that we DO have this asinine paradox where it is OK to be fat but not to be called fat, the breathtaking lack of tact and judgment on behalf of the staff member is entirely regrettable. But Jeepers Creepers, people do a lot worse on a daily basis with no consequences whatsoever. Firing somebody is excessive; a reprimand would be appropriate.
If the guy does it again, sure, fire his ass, but not on the first "offense".