The meal portion may seem too big because there are a lot of fibre-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 litre).
However, if you feel uncomfortable eating so much, focus on your feelings and not force yourself to eat the entire portion.
Please remember that if you didn't finish your food at lunch, you can't compensate for this meal later on (with some snacks or a larger portion of dinner). You should continue eating with the plan. If you are hungry, drink tea, and water.