Before we begin
Essentially, Kolkata Biryani is much lighter in the notes when compared to its Lucknowi or
Hyderabadi cousins. The reason is simple - The humidity of this region makes it impossible to digest anything very heavy, and the king's cooks were worried about the emperor's health. Kolkata Biryani features the Potato, once an exotic ingredient was introduced to the Nawab's kitchen by the Portuguese, and the chefs used it because it was cheap and not familiar and replaced a considerable amount of the meat. The meat and the potato makes this biryani more carb-heavy, and therefore, heavier in calories than the Lucknow/Hyderabad counterpart.
Kolkata Biryani, like Lucknow or Hyderabad versions of the Biryani, is a version of the Awadhi biryani, adapted by the cooks to suit the local temperament. I have asked a number of people over the years, and they agree that while "Awadhi" is more of a theory, the different versions of the biryani is more like the adaptation of the theory for practical solutions. So, under the broad spectrum of the Awadhi Biryani, you would find a number of adaptations - in different cities, according to the tastes of the locals.