Both my daughters have a very different body type from mine. I have always been skinny even when eating junk and not exercising. They, on the other hand, are very much the curvy kind. I have emphasized and modeled the importance of taking care of one’s physical and mental health by eating a balanced, healthy diet and moderate quantity of food, and keeping fit among other things. I have always spoken positively about their body type and told them that it doesn’t matter what you look like, but that we owe it to ourselves and our loved ones to take good care of ourselves.
As others have said, the weight gain may be due to many factors. I would be kind and non judgemental, I would not discuss or focus on the weight gain itself necessarily but I would discuss those factors that directly impact weight as a measure of health with them, because I am their mom and I care, and because they would not resent that. You will have to judge how it would work with your child or what other approach would work better for yours.