Congratulations on your surgery. Mine was 4/3/03 and I've lost 72 pounds. We're planning our trip for October 7-14 and this was a concern of mine as well. From my research, I've decided not to expect Disney to make special concessions for my small appetite. The buffets are definately a huge waste of money and we won't go to any. At the sit down and counter service places, I plan to either order off the child's menu (if there's an acceptable choice), order an appetizer (again, if there's something appropriate), or split an adult entree with my DS3 and DS1.
Although, I understand you can attend a character meal and NOT eat anything. In that case you would pay only a modest "entertainment fee" to cover the cost of your interaction with the characters. I haven't yet tried this as its not quite time to make my PS.
Hope this is helpful. Let me know how your experience turns out. You can PM me after your trip if you like.