The causes of naturally thick legs are as follows.
Inter-individual variation can occur and is influenced by genetics, some people have large bones and muscle mass in their legs and can appear to have thicker legs, these are normal.
Some people are more prone to obesity due to metabolic features and can also appear to have thick thighs, which can be reduced by controlling diet and increasing exercise to achieve total body fat loss, thus reducing leg circumference.
Sitting on one’s knees since childhood can also lead to thick calves.
Some rare genetic conditions such as Duchenne-type muscular dystrophy can also manifest as pseudomuscular hypertrophy of the legs.