How can you explain the fact of the existence of people with different body builds (thin and overweight)

How can you explain the fact of the existence of people with different body builds (thin and overweight), which practically does not change with the intensity of their nutrition?

Composition is affected by various diseases and metabolic disorders, as well as differences in physical activity.
Example: an athlete eating the same amount of food with an ordinary person will have less weight, because sport requires a lot of energy, which means that his daily need for food to maintain real weight will be higher.