Big leg muscles are gained through anaerobic exercise. This type of exercise activates the fast twitch muscle fibers. This will build leg muscles that look bigger and more defined. The shape of the muscles of the legs will be more pronounced. Primarily you will want to lift weighs with your legs and there are a few exercises that can do this.
Sprinters like Usain Bolt have many fast muscle fibers as they need that explosive sprint and you typically can't do this type of exercise for very long. Exercises will be squats, leg curls, leg extensions and calf raises. You go with the a heavy weight so you can only repeat the exercise 8 to 10 times and for 3 sets, 3 times per week. Over weeks of doing these exercises you will be able to do it with heavier weights.
A treadmill can help build leg muscles but you won't be doing endurance runs you will be doing sprints and other interval training. I have devised a sprint routine through that link. You may also want to try high intensity interval training. A combination of weight training and a treadmill workout will increase the size of all your leg muscles like the quads, hamstrings and calves.
If you're a female and you're worried about getting freakishly large muscles, you've no need to worry. Females don't produce enough testosterone for your muscles to get very large. To summarize you need to build endurance on a treadmill to really help you get lean and toned legs. Also lose weight if you need to which will help improve the shape of your legs especially round the hips and butt.
Big leg muscles require explosive and short exercises which will feel hard to do but the results will be rewarding. Which ever you want to do, remember an exercise routine is very beneficial to your health so stick at it and you will feel wonderful. The shape of your body and your confidence will vastly improve. It's really not that hard to do and shouldn't cost you too much either.