All runners have different builds. There are runners with thick legs and there are runners who are rail thin. Your thighs will not get bigger simply from adding running into your exercise routine. You can achieve the look you desire by following the appropriate running program.
Your genetics and bone structure determine the shape of your legs. Running will not make your legs longer or turn them into tree trunks. The amount of muscle and fat that you have on your legs determines how your legs look. The genetic makeup of your body determines where you store fat. Some people store more fat in their upper bodies while others store more fat in their midsections.
Running for distance can create a sleek body with lean muscles. Running blasts calories and melts fat. Running can reduce the size of your thighs because as you run, you burn fat and build lean muscle. The more muscle you have on your body, the more calories you are able to burn during and after your workout.
When you sprint you take bigger steps and maximize your muscle power. Sprint training builds muscle so sprinters often have a bulkier appearance. You can gain bigger thighs if you train like a sprinter, using jumping and plyometric exercises to enhance your sprints.
Interval training training is a type of exercise in which you alternate between fast sprints and slow recovery run. This type of training uses both the aerobic and the anaerobic energy systems to increase the heart rate. The Mayo Clinic explains that interval training burns more calories that traditional cardio, so it can shed fat fro your legs. But it does not emphasize as much muscle building as sprints alone, so it won't bulk your legs up either.