This will avoid targeting the type of muscle fiber that makes muscle get big and bulky. Instead, long-duration cardio (e.g. long-distance running) will target a smaller muscle fiber that has very limited capacity for growth. The result is that you build stronger muscle without building big thigh muscles, while burning fat. On the other hand, avoid short intense bouts of cardio (e.g. sprinting, HIIT). Although this can be a great fat burner, it may also increase the size of your thigh muscles. Endurance running is one of the most effective exercise to get defined, slim thighs and legs. Endurance runners tend to have very slim legs, especially in compared with sprinters who have highly developed and very large thigh muscles.