It is sad for your older daughter, but this is likely going to continue throughout their lives, and not just from their grandmothers, but at school and everywhere else. It's just a fact of life that cheerful, bubbly, attractive, slender girls/women get more attention and accolades than "chunky", argumentative ones, who are often bitter and resentful because of a lifetime of the different treatment.
I certainly got more attention from my grandparents than my older sister, simply because she was shy and was like talking to or hugging a wall, and didn't seem to have much personality, whereas I was always full of hugs and chatter.
Life isn't fair, and it really isn't fair for parents to try to raise their children to think that it is, because it isn't. It sets them up for a whole lot of disappointment and anger in life. It's a good idea to teach simply that life isn't always fair, but it is what we make of it. We can choose to be bitter about it, and dwell on it, and make everyone around us (the grandmothers) miserable, or choose to make the most of life and be happy. They probably don't realize they are doing it, as they deny it. And, being overweight as a child is a legitimate concern, even if it seems due to genetics (genetic sweet tooth, even). Diabetes is no picnic, and her grandparents don't want her to deal with it. Also, younger children are cuter; there's no denying that. When there's a baby and a toddler, the baby gets more attention; when there's a toddler and a preschooler, the toddler gets more attention, and so on. I'm sure some of it is just that.
I have relatives who think that they haven't gotten as much attention or gifts or whatever as other members of the family, and it's a real pity. They dwell on it and pass it on to the next bitter generation. After a while, that's all they have to talk about, how they've gotten the shaft. Frankly, who would want to spend much time with that poor, wo-is-me attitude. I think you'll be doing your older daughter a real disservice to raise her up to be comparing herself to her sister, because no matter what, she'll never feel like she measures up. Don't do that to her. Teach her instead to blow it off, and again, that life isn't fair, oh well!