Don't bother - The ignorance of the general populace is astounding. Are you satisfied with yourself? Good, then carry on and leave the ignoramuses to do what they do best.
tell them not to be hatin... or judgin'!!
First, I would say is if it was me, the only person I would care about enough to tell would be a friend and they shouldn't care and/or they surely wouldn't make fun of you.
Second, let criticism strengthen you against idiots who are too shallow to care about the reason they are so clueless about the truth. Then again, you could take their weird look as a clue to educate them on a new way to think about slim people. Who knows you may make a friend or two.
***Btw this is one way to get exactly what you are asking since facial reactions aren't an issue here. You may have to deal with rude answers though but that is another subject.
I hope that helped. Blessings to you. :)
dont tell them anything...just eat in front of them :)