I'm so sorry to heat what happened. You gotta remember the source of the comment. Kids are brutal, period. What the child said was most likely what the parents think and have said as well. They are walking recorders, remembering and repeating everything that is said. More times than not, they don't even realize what they are saying, much less realizing what they say is rude, crude and totally unacceptable.
Please don't let this get you down. You come a long way, and have every right to be proud of yourself. Me, I would write a letter telling the teacher what happened and suggest an "intervention" of sorts talking about obese people.
Or, you could have talked to the kid at their level "I may be fat, but your ugly and I can diet." (sorry, couldn't help that one!
In all seriousness, I would take some sort of action that will help educate. Perhaps you could have the teacher invite you in to talk about it?
Whatever you do, don't let it get to you. You've come way to far now! Get yourself around some positive people, and use positive self-talk!