Although you do not eat, snack, or drink all day, you still have to keep the portion of the meal during meal and breaking the fast. Eating with excessive portions can increase blood sugar abruptly. Whereas as mentioned earlier, the body does not produce much insulin when fasting. Sugar will only be converted into fat in the body.
In order not to overeat, try using a smaller plate size. You can also eat or break the fast with food that can make you quickly satiated like soup.