The way the body responds to food is much more complex than the way a calorimeter does. For example, having lipids (components from fat) in your small intestine reduces your hunger, because your body knows you have a lot of good energy coming up. Sugar and other low-complexity carbs, on the other hand, is quickly absorbed, and does not do much to reduce hunger even in the short term.
Even different sugars (glucose/fructose/sucrose) affect the satiety response differently.
Are you honestly implying fats aren’t easy to gain weight on? There’s a reason arctic explorers packed lots of butter and oils. It’s a super efficient way to store energy and consume it.
I eat lots of healthy fats and no sugar or wheat. I'm borderline underweight. When you eat such a diet, your insulin levels stay low. High insulin is what causes your body to convert sugar to fat and store it. Low insulin causes your body to burn your fat reserves. I just got back from a 5 day, 50 mile backpacking trip where I averaged about 1000 calories a day. Not once was I hungry.
The arctic explorers weren't gaining body fat from the fats they ate, they were burning them almost immediately.
We are learning more and more that gaining weight has more to do with insulin response and various other super complicated pathways -- not simply "calorie in = some weight gained". If that were true, people wouldn't lose weight on near/full ketogenic diets high in fat.
No, I'm not. I am saying that humans are complicated, and that the effect of food on weight is not simply down to "how calorie dense is this substance." Unless you are force-feeding a person into obesity, other factors beside caloric density matter a great deal.
You may want to do some reading on the satiety response. You could also imagine whether you think you would gain or lose weight over the next month on a tallow-only diet.
Even different sugars (glucose/fructose/sucrose) affect the satiety response differently.