Q: How would you lose bulky leg muscles the fastest?

A: I, of course, haven't seen you, but chances are very high that what you think is bulky leg muscle is actually mostly fat. Very few people who haven't weight-trained (and this includes women) actually have so much muscle mass in their upper legs that it gives them bulky appearence. For most what is perceived as muscle bulk is actually fat. Test it by poking your upper leg while flexing it -- you should be able to tell very easily by how jiggly and soft the tissue is. Muscle tissue should be quite hard.

If you determined that it's really mostly fat that you have there (which I suspect is the case), then you need to lose fat overall in order to lose it in your upper legs. Please read this to learn the most effective way of losing fat.

If you found that it really is muscle mass that's making your legs look bulky and you are certain that you want to lose it, then the best bet is to go on a calorie-deficit diet while NOT exercising lower body (either weights or cardio). Any diet will make you lose muscle and a badly constructed one will make you lose a lot of muscle. I do NOT recommend doing it, even if you're sure that your bulky legs are due to too much muscle simply because losing a lot of muscle will almost inevitably lead to more fat gain in the long run and worse situation than you when you started. But it's your choice ultimately. Just be REALLY REALLY sure that it's muscle and not fat that's causing your problem and be prepared for some fat gain when you finish with your diet.