If I were the hostess and knew one of my guests could only eat an appetizer & salad, while the other guests ate a full meal, I would be uncomfortable. Most people will tell me what is going to be planned and if it looks like i will get just a side or salad, I will also offer to bring something for my main course, but will also bring enough for all to have some if they want. If that is turned down (OH, you don't have to, there's plenty of food here!) I just decline the invitation. I chose to eat this way and I am not pushy about it , but if I am going to be the only one not getting the 'meal', it can be awkward with both guest and hostess feeling weird. Nothing like everyone leaning back and saying their full while all you can think about is going home and making some pasta & veggies!