Not all fats are created equal. And depending on what type of fish you're using, fish are often very fatty. Even more so than pinkies. Salmon is very fatty. Silversides are too. They're just a type of smelt. The native Americans around here used to dry them and use burn them like a candle. They're that fatty. I don't think it's so much about the fat content that makes snakes overweight anyway. Pinkies are simply very dense food and if they're being overfed on pinkies, their body mass reserves are going to max out quickly especially since captive snakes don't have to expend any energy to find the food.