Actually, its all about the energy in the food that matters. If you insist on eating a lot of food the body will grow in weight and if you eat less you won't.
Short term it will be stored as sugar in the blood and long term overconsumption will be stored as fat. So celebrating one day and eating alot of food won't make you increase in weight. But long term overeating will.
Diets like 5:2 uses the 2 days of not eating to cleanse the blood from extra sugar - which are consumed the days of not eating alot.
Atkinsons diet uses the fact that protein rich food contains less energy to get people to eat the same amount but consume less energy.
Its not magic. Its just pure science. Or simply put: Shit in, shit out.
Those using sweetener is probably so addicted to the sweet taste, that they consume alot of other products which give that taste. Thust they overconsumate on food and get too much energy. So it can be better to altogheter do away with the need for sweet food.