I think Grandma needs a copy of the ABA's booklet for grandparents. Or, perhaps just a lesson in "How to keep your nose out of someone else's business"?

I would respond that you don't appreciate the comments, that you and Ava are doing very well, thank you, and hasn't she been paying attention to all the recent media coverage about overweight children and babies? I'd then ask her if she is on a diet to lose/gain weight, as you noticed she wasn't looking very healthy either.