The primary factor influencing whether an exercise builds muscle or not is the rep range that you use. Losing fat depends heavily on nutrition. If you want to get stronger without gaining much muscle mass, then use a range between 1 to 3 reps where you reach muscular failure in that range. Alternatively, if you use a rep range of 15 or more reps per set, you will build muscular endurance and very little muscle mass. 

You can combine both of these rep ranges into a workout while avoiding everything in the middle (Between 4-15 reps).

Exercises such as squats, lunges, leg curls, stiff-legged deadlifts, leg extensions, and calf raises are all excellent for toning your legs.

Cardiovascular training such as running is very good for keeping the thighs from gaining much muscle while still losing fat. Not too many runners have big legs but most have very lean legs. The same is true of cycling or the stair machine.

Here are some tips for targeting your nutrition for fat loss:

1. Increase your protein intake, e.g. eggs, lean meat, chicken, fish, etc. This helps support your metabolism.

2. Cut back on junk food. This will make a huge difference quickly. You don't have to totally eliminate it, just decrease it greatly.

3. Don't eat anything before a workout. If you do, your body will burn what you just ate rather than fat.

4. Eat about an hour after a workout. This allows your fat burning to continue until you eat. Don't wait much longer than that though, as you need food to recover. Stick to protein and low fat foods.

5. Try not to eat late at night. Your body won't have a chance to use those calories and will store them as fat.