James, it really is hard to know how to handle people like that. Sometimes they are well-meaning, just dumb about how they show it. Other times, they're just plain jerks and not worthy of a second's notice.
And that's the thing. You know all that you are doing, so reinforce that within yourself. If you are confident that you are doing all that you can, then with that knowledge you can withstand anything and let it wash off your back. Not always easy in practice, but with practice it can help.
That said....
I would be tempted to look them in the eye and ask them how big their feet are (or how tall they are, or what their hat size is). When they respond, no matter what they say, ask them, "How could you let your self/feet/head get so big?"
The way I see it, if nothing else they'll either be too confused to respond, or if they are truly well-meaning it might prompt them to ask what you mean. At which point, you can let them know that you have about as much control over what your immune system is doing to you as they had over how tall they grew. It doesn't have to be a combative exchange. Calm is better in this case, I think.
Anyway, that's what I think.
Many hugs,
