One of my granddaughters is overweight, too, but I don't dare say anything because I've found my children DO NOT want my parenting advice and it has never gone well when I've offered it. So what do I do? Shut up and love them as they are.
It's some consolation that I was a chubby child, made fun of in school, so grew up with a resolve never to be heavy again and I've always been slim as a grownup.
When my own daughter was heavy, I encouraged lots of outdoor activities. Maybe you could introduce your granddaughter to some activities that will get her mind off food and on doing things.