Tapioca is used as the starch source in some "no grain" pet foods. Tapioca (cassava root, manioc) is first and foremost a source of carbohydrates, but very little else. It is actually a nutritionally poor substitute for grains. And as you know Carbohydrates are usually not a good thing and promote weight gain!!
So if tapioca is high on the ingredient list there is a large amount of this ingredient in your pets food.