My mother said similar, GrandTheft, about my 13 year old when he was maybe a year old: "ooh, we'll have to watch his weight and put him on a diet when he's older if he doesn't lose those chubby cheeks!".
He was the spitting image of me at the same age, chubby cheeks included - and suddenly the reason behind my lifelong eating disorder became clear... although as my mother's "fond" name for me at that age was Pig Face, I'm actually horrified at my own lack of self-awareness concerning that.
My son is now a healthy weight for his height, eats like most teenage boys do (ie, he only has to look in the direction of the 'fridge and suddenly its empty!), and has very little time for my mother. You want a healthy child, not a "skinny" or a "fat" one (God, how I hate those terms!), and as long as your child's GP/HV think she's healthy, then that is what you need to keep focusing on. Not your mother's out-dated ideas about food and weight. At the very least it says an awful lot more about her idea of being a good parent, than it does about yours.
though, because I know exactly how tough it is.