I don't think concern over calories and nutrition limits what we can offer. If anything, it pressures the creative side.
While reading this thread, I'm reminded of the long, multi course meals of years past. In the Epicurean by Charles Ranhofer, the menus have six to eight courses. Surely the portions must have been smaller than what we serve now. Of course now we have tasting menus with that many or more courses but with really small portions and very high prices.
I wonder how you get customers in a mid level restaurant to order more than just an entree if the portions are so big? We also seemed to have moved away from devoting the time to multiple courses. No one seems to order a soup course, a salad course, an appetizer course, then an entree, then a dessert and no restaurant seems to offer that kind of meal experience. All of that would take time to enjoy as well as requiring smaller portions for each course as well as occupying a table for a much longer period of time.
Now it seems the general expectation on both sides is get in, get stuffed, and get out.
If a restaurant offered multiple courses, each made of healthier choices and smaller portions, how would you get the general public to order multiple courses so the house makes enough money to allow the table to be occupied longer? Do you do a la carte or prix fixe? Most menus have soup, salads, appetizers, entrees and desserts on their menus but few customers seem to order all of them at any meal. Is cost the deciding factor or is it knowing there will be too much food?