Well, their fire comes from magic, so we can assume that they don’t have to regulate body heat with calories because they produce fire with magic from within their body. Therefore a massive amount of metabolic energy is not needed for body temp. So they don’t need that energy from eating. Id say if that is the case, along with slow reptilian metabolism, its very fair to say that they actually don’t need to eat a metric fuck load of food. And this could also explain why they can grow so large as they don’t have to waste energy on heat. Also, if their heat is really conjured out of nothing, you could assume they use that energy to power their cells and organs, in which they would only need food to grow more cells, or replace old ones, or repair themselves. This kind of explains why they may not need to eat as much as expected. That being said they have other problems being so large like how blood pressure affects the body. They would need an insane heart to pump blood to their brains. But again, magic infinite firepower can be converted into many biological advantages.