I recently attended a social event where I overheard someone greet an elderly woman by saying, “You’re so skinny. Are you okay?” The expression on the elderly woman’s face was one of shock and she was obviously at a loss for words. I perceived the comment put a damper on her entire evening.
It is impolite to comment on the health or specific physical changes of another person. You may think the comment is positive, “Wow, your weight loss looks great”, but the other person might think you thought they needed to lose weight. The same rule applies to hair styles, hair colors, and cosmetic surgery. “I like you as a blond”, could be interpreted as you didn’t think they looked good as a brunette.
If you feel the need to make a positive comment about a physical change, keep the comment general. For example, “You look wonderful!” Otherwise, keep your comments to yourself.