An overweight person can be eating caloric diet equal to a skinny person and not only not lose any fat, but also be painfully constantly hungry, and be technically starving themselves as their body leeches what it needs to run from their vital organs.
Meanwhile a fit person can be eating excess amounts of calories, but because they eat minimal carbs/sugars their body doesn't hold onto the extra calories they are eating (mostly eating fats, veg, meat) and they more easily lose body fat, and not only that but be constantly feeling full and never have hunger on their mind.
Obesity is an inflammatory disease. It's not as simple as the amount of food you eat irregardless of what you eat. What you eat is extremely important, and when you eat right portion size is less important because your body won't allow you to eat more than what you need.