The meanest thing you can say (and I've used this a lot when someone calls me tiny) is to simply comment that you're not tiny, that you're the correct weight for your height.  You're only commenting about yourself, so they can't really bitch about it, especially since they started that particular conversation.  At the same time, you're pointing out that they're both morbidly obese and extremely stupid since they can't even recognize what a healthy body is supposed to look like.