You meet a overweight person and tell them that they are overweight. By saying that, what are you trying to achieve? To motivate them to exercise? This objective cannot be achieved because even if it is the truth, it will only demotivate them and hurt their feelings. Calling someone overweight can never be recalled as polite. It is just rude.
Who would like it if someone says “Oh, you are fat?”. You are only just addressing the ‘problem’, not a solution. Those who say this kind of things are unlikely to have good intentions. Victims of such verbal abuse will feel insecure about themselves and feel ‘ugly’ and scared to go on public.
There are other ways to giving someone constructive feedback. You have to be cautious of every word you let out. Don’t give feedback for the intention of destroying their dignity. Treat them like a human. Not a emotionless bag. You need to choose the right words before helping someone out. Make sure your intention is pure though.