The meal portion may seem big because there are a lot of fiber-containing foods in the dishes (for instance, vegetables are very low in calories, and to get the right amount of calories, you need to eat a rather big portion).
A portion of 500-700 grams is quite normal for a good healthy meal (the volume of our stomach is 1 liter). However, if you feel uncomfortable eating so much, focus on your feelings and don't force yourself to eat the entire portion. You would have larger portions if you chose three meals per day. If you want smaller portions, you can switch to 4-5 meals a day; snacks will be added, but the size of each portion will be smaller.
Please, remember, if you didn't finish your food at lunch, then you can't compensate for this meal later on (with some snacks or a larger portion of dinner). You should continue eating according to the plan. If you are hungry, drink tea or water.