Not all calories are created equally, and they may function differently in your body, making the amount of calories you intake not the be-all, end-all for weight loss.
For example, fructose, a sugar found in fruits, honey, high-fructose corn syrup, sauces, grains, juices and sodas is stored as glycogen (storage form of sugar) in your liver.
If the liver is full, the excess fructose gets turned into fat, raising the levels of the blood sugar balancing (and fat storage) hormone, insulin.
So, 100 calories of fructose may increase fat storage and insulin levels, leaving you starving and fat.
On the other hand, there’s protein found mostly in animal meats, beans and legumes, and nuts and seeds.
Protein (unlike fructose) increases metabolic rate, lean muscle mass, and keeps you feeling full- no willpower needed.
All that to say 100 calories of fructose acts completely differently than 100 calories of protein.