I read this yesterday and felt so bad for the people who had this happen to them. I was teased a lot about my weight when I was a child and so I am super sensitive to people being mocked for their size (whether it's thin, heavy, tall, short) and will generally defend them if I overhear comments being made.
Last night, I kept trying to think of EHell approved responses that someone who is the target of such mean comments could use.
The reason bullies do this is to make you feel defeated and look sad so they can feel better about themselves. You have to do the opposite of looking soul-crushed because then the bully feels they've failed and isn't rewarded for their mean behavior.
I like the PPs suggestions of laughing. That's ideal, but very hard to pull off. If the offender is driving away, it's probably best to pretend that you didn't hear them at all, so they think their efforts were in vain.
But I think if they're still within earshot and there are witnesses, you want them to feel mocked instead. The best thing I can come up with is to say confidently "Gee, that's not what you said to me Friday night when you were trying to get me in bed." That'll get everyone laughing at the bully, which is the best way to stop that behavior and restore some of your self-confidence.