One of the reasons that fancy or gourmet restaurants serve smaller food portions is because of the pricing of the ingredients. The more high-priced ingredients they use for the dish, the more expensive the dish will be, the reason ingredient are perched at such a high price is to ensure they source and purchase the very best of each product to serve to their customers.
Another contributing factor is the idea that food should be experienced. The small bite you have should provide enough flavour and succulence that you are enjoying your food, not just chowing down on large and unnecessary portions with little flavour or luxurious.