When more calories are consumed than the body needs, it is converted into fat. Initially, fat cells grow in size. When the size limit is reached, other cells begin to form. In this way, fat cells grow and obesity occurs. When weight loss occurs. At that time the size of the fat cells decreases. But the numbers are not diminishing.
The energy that the body needs to function comes from calories. When more calories are consumed than the body needs, they are stored and obesity begins. Obesity is more common in sedentary workers.