You state that it's difficult for a fast food restaurant to manage a la carte purchases when that's exactly what they are designed to do. Their entire business model is based around an a la carte menu.
You then justify larger portions as a means to mitigate customers ordering additional portions. The reality is that for most casual or low end dining the customer knows what to expect and orders appropriately. At best your assertion about large portions would only work to satisfy first time customers.
Fast food can bring the total cost closer to the raw ingredients by increasing the utilization of real estate, capital, and labor. A fine dining restaurant needs to dedicate much more real estate and staff to each customer, since they cannot serve as many customers in one time period. At the same time, they probably need at least some higher cost ingredients to satisfy the different expectations of a customer willing to pay higher prices.