Constructive criticism implies you are attempting to "tear something down" in order that it can be replaced by something better.That means the originator was wrong or at least not optimum. Your endeavor is not only to convince someone that they are wrong but you are also correct. The person usually has a personal investment in his choice. A touchy situation. Often the perso is just plain wrong. That is pretty easy. More difficult is where the person has made a correct choice but there may be more effective alternatives.
There is a difference between caling someone fat and suggesting an improvement. If you lost weight you would improve your appearence, avoid disease, increase your enrgy level and enjoy life more. While the first statement is accurate. The second is more palatable and more likely to yeild a positive result.