I don't think lentils are the problem, it must be whatever else is in that soup... (I didn't google the recipe).
I just made my first lentil soup this week, too. But the soup was incredibly low calorie. It had a cup of lentils in the whole soup; a cup of lentils has 230 calories and I got 8 servings out of the soup. So... the lentils contributed 29 calories per bowl. The other ingredients were vegetable broth, spices, onion, garlic, and celery.