If you have slim calves and take up running, you will likely develop muscle, which will make the calves bigger. On the other hand, if you are carrying extra fat when you start a cardio fitness plan, such as running, then your calves may reduce in size. Running is a vigorous form of cardio that activates and relies on your glutes, quadriceps, hamstrings and calves to propel you forward. As such, running is an efficient calorie burner; running at the pace of 6 mph can burn approximately 405 calories in 30 minutes, according to Fitness Magazine.
Located on the back of your lower legs below the knees, the calves are a two-headed muscle group consisting of the gastrocnemius and the soleus. The gastrocnemius is the larger of the two and makes up the bulging shape of the calf. The soleus is seated deeply in the calf under the gastrocnemius.
Working together, the calf muscles serve to lift your heel when your leg is both straight and bent. An integral part of the running motion, the calves initiate the foot cycle and power the push off and have an effect on both your control and speed. Repeatedly engaging the calves in this manner helps to build and develop the muscle group.