Although not quite as dedicated as you Forzagto, I am a strong proponent of healthy eating and in general stick to low fat meats, lots of veggies, whole grains, moderate alcohol, etc. Also, incidentally, for training benefits. That said, I also am addicted to delicious food, so I make a compromise when I go out. Especially in the case of a special or celebration meal, giving yourself up to the skills of a great chef can be a wonderful thing. After all, its only one meal. You can always go back to steamed tofu and kale the next day.

In fact, in my younger days I was a more dedicated distance runner and every week that I ran 50 miles, I'd treat myself to a pint of Ben and Jerry's at the end of the week. Then again, I was 6' tall and 155 lbs, so the extra grams of sat. fat were not a concern.
So my personal recommendation is to accept the meal for what it is and enjoy the experience of something new and different. That is what these meals are about. If that's not up your alley, than stick to something else.
As far as portions are concerned, of the best prixe fixe menus I have tried in town, I left Trotter's and Alinea (12 course tasting) not entirely full. In both cases I think I had to down a sandwich (whole wheat bread, turkey breast, no mayo) when I got home to fill myself up. The food was excellent, but I'm just saying...I needed a snack. Avenues filled me up, but the chef (Graham Bowles) has since left and now starting his own place (Graham Elliots) to open soon, so I can't speak of the new chef. I've already mentally lined myself up at his door in anticipation of the opening. Schwa I don't really recall, but I think I left full. Then again, my wife wasn't a big fan of some courses there, so I really had a meal and a half.
Not that portion size is the reason for having these types of meals, but you did ask.