Hi Alison,
I experienced nearly the same thing w/ my MIL, and in our case its more cultural than anything.
My son is now 6 and has a slight build, purely genetic from my side of the family, which should serve him well later in life. He's consistently been @ the 50th %ile in weight since birth and 90th for height, making him taller for his age & lean. In her defense, in my MIL's culture/homeland, if you don't have a roly poly baby, it may not survive!
But that doesn't make it easy to handle the chatter sometimes. You could try bringing her with you to the next dr appt to hear for herself.That was helpul in my case. You could also give her some kind of a schedule when she's caring for her to see if that @ least makes her feel better.
Beyond the food, no parent can ideally do everything they want to for/with their kids all the time. That's what grandparents are for! Suggest that reading together can be one of their special activities together and will augment what you ____@____.com luck. I know how aggravating it can be, but also remind yourself how lucky you are to have them nearby and involved for the extra support to you and their relationship with your daughter.