How would you address your mil with this?

*Dawn*
I have a solid kid. He usually gains weight and then shoots up in height so it balances out. Well, over the summer we noticed he didn't do much growing, but has gained some weight. When we noticed about 2 weeks ago we implemented family walks in the evenings and getting him outside more for activities. He doesn't over eat so we don't regulate/restrict his food, but we make sure he upped his water intake. My husband was a very hefty kid, but of course my mother in law saw nothing wrong with him when he was little. When she sees my son it's a whole different story. She wanted to take my son for the day to spend time with him before school starts. My kid comes back and tells me she told him. "Too much sugar equals a fat kid" when he grabbed two pieces of candy corn from a candy dish she has at her house. I am livid because I struggled with food as a kid and comments from adults always hurt the worst for me. I told my husband to address it with his mom since it is his mother and if it was my mom I would be the one handling it. Well he acts like it's not a big deal so I will be contacting her tonight when I am off work. I'm not going to be a bitch about it, but I just want to explain to her my struggles growing up and how small things like that stick with kids. If she wants to say anything to him about food it should be something a little less harsh. He is 6! I don't want him ending up like me and struggling with food because of careless things adults say. Any other moms ever go through anything like this?
*i know I'm going to probably get comments from people saying "oh well the truth hurts". That's the thing, comments from strangers I would tell him to let roll off his back...it's harder to do that when you get idiotic comments from the people that love you the most.