Wow what a rich and diverse group we have. So many helpful comments.
For me, what gets me all the time, ppl who haven't seen me often, will say, oh wow, you've lost a lot of weight, I smile and say, no not at all, I just think you remember me heavier
OP if in your judgement this ole gal isn't suffering from some sort of mental impairment, then there were lots of great options offered, that could fit into your own personal style.
For me I like real humour. but that does depend upon if you have found the ridicuously funny in the situation. I don't know what it would be for you. But there is an element of slap stick, as this ole gal every time she sees you, comments on your weight gain.
I too was raised to respect my elders. But also to honor them. After all someday we will be there. Of course it is closer to me, than to many here on the board.
What I think is you truly think about the situation. Try to see it from all points of view. Then select and preplan your response, that meets with your values and style. And you have many great responses here as well