Endurance running is especially effective in those who have big thigh muscles, as it may decrease the size of the muscles and reduce fat around the muscle to make the thighs smaller. It will completely transform the size and make-up of your thighs and your calves. Therefore, to slim down thighs do endurance running with little or no incline. If you find it too difficult to run or are unable to run, try the elliptical trainer with little or no resistance. Posture on the elliptical trainer is critical. Correct posture will target your butt, outer thigh and hamstring muscles (back of thighs) and lessen the strain on your quadriceps (front of thighs), avoiding you from inadvertently building big thighs.