The Eye of the Beholder

I saw a friend recently, who I hadn’t seen in a very long time. She had gained some weight, and looked better than I had ever seen her in years. She looked healthy and happy and grown up, and like she had her shit together. She was glowing, and smiling, and there were no bags under her eyes, and her hair was shiny, and I just thought she was so content with herself.
But it’s socially unacceptable to comment on someone’s weight gain, so I couldn’t even pay her that compliment.
Why do we find it okay to delight in someone’s weight LOSS, but taboo to mention their gain? I have another friend who would look much better with some more meat on their bones, but more importantly, they would be much healthier, and for whatever reason, they are tiny. They can’t gain weight, even though they want to. And you know what? I constantly hear people complimenting them on how good they look. It drives them crazy.
Another friend is considered “obese” but is happy, gorgeous, and healthy as a horse. Her family is always asking about her dieting, even though she eats like everyone else, and never mentions dieting or weight loss.
We get so fixated on other people’s body type, that we let our own discomfort about their body take priority over their comfort and preference.
I wish my friend could hear some positive reinforcement about her health. How do we approach conversation that goes against our social norms?