1. The person might have lost weight as a result of depression or an eating disorder. Not exactly something you want to discuss in mixed company.
2. If meant as a compliment, the statement implies that the person was overweight before. Thanks A LOT for noticing.
3. If meant as concern, it implies that the person’s current weight is unhealthy or unnattractive. Again, THANK YOU.
4. It means that you are noticing the other person’s body, which might make him/her UNCOMFORTABLE. Please STOP LOOKING AT MY BODY.
5. Most importantly, it is NONE of YOUR BUSINESS.