Disney has no mechanism in place for people who want to eat a smaller meal. It is what it is and the prices are the prices and most restaurants are booked up well in advance.
As others have pointed out kid meals are an option (but imo often blech and not something I'd want to eat), or ordering apps as your main (much better options), splitting, or just eating what you want and leaving it. I will be doing a combo of these on my upcoming trip.
Liberty Tree Tavern is one good option to order kids meals because they have both turkey and pot roast. Sci Fi also has a kid size cheeseburger. Teppan Edo has kid portions of the main meals. I'm sure there are others but those are some on my list where the kid meals are kind of just smaller versions of what is offered on the adult menu.