This is a business model. 

In Brazil we have a sort of self-service restaurant called per-kilogram, where the plate is weighed and (discounted the plate) you pay proportionally to the weight of food. Each restaurant fix a price per kilogram of food, carefully averaged to make the business profitable (after all, meat is more expensive than salad). 

But this sort of business has a nasty side: they do everything to encourage the customers to consume larger and larger portions of food, in order to increase the net profit. One of such things is to use giant plates, much bigger than the ones you would use at home, so that people are fooled into thinking there is little food served.

Do a restaurante in this fashion, but give a discount if the weight of food is close to the recommended weight defined by health agencies. In fact, the lowest a customer can pay should be exactly when he eats the recommended value. An average number need to be carefully studied, and whether it is necessary to know the person's age or other data. Also, punish in price if you eat more than the recommended weight. 

The idea is to promote healthy eating habits. Give your regular customers a goal to work towards. I don't know about you but money can be a powerful motivator.