My grandmother as well, and it was always my weight. Growing up, I swam competitively. I was never, and never will have, the body of a supermodel. I'm just not built that way. so when I graduated from college, and started gaining weight, she offered me a sum of $$, pretty decent, if I'd get down to x weight, a very specific number. Never mind if i were one or 5 lbs past that, and still looking good; no, I had to weigh that since that was the heaviest she had EVER been, and as were were the same height, but nto the same build, she felt that was the ONLY acceptable number for me.
I pretty much ignored subject when around her. I also had to fudge on clothing size; she was very generous and would sometimes buy me clothes for work, if she liked something, and I did too. But i could never tell her what size it was, say a 14 rather than a 10 or 12, as that wouldn't do at all, due to her ideas about weight etc.