Any fast food in the body is processed into glucose (sugar), and in general any food in the stomach turns into glucose ... the more we think, the more sugar ...
Therefore, if there is a lot of sweets, then without processing it will immediately be deposited in the body in the form of fats (who are inclined)
If you are not inclined to weight, then eat as much as you like
Nowadays it is increasingly said that the source of calories is much more important than their quantity. Sugar has great effects on the human body. Watch the documentary "Sugar". There, the Australian puts on an experiment and every day eats 40 tablespoons of sugar, dissolved in "low-fat" and "healthy" foods.
Only if the calorie content of sweets is large enough so that its surplus accumulates in the form of fat mass. If you do not go beyond the normal range, then you can not gain weight, eating only fast food and sweets, which is not so easy.