Do leg lifts slim your thighs?

Leg lifts in and of themselves are not going to slim your thighs. They will help you to burn more calories primarily.... Secondarily, they will 'tone' or build a little bit of lean muscle if you are eating at maintenance or a slight caloric surplus...however not much muscle as you are not doing any resistance training.

Depending on what kind of leg left you are doing, you will be working your abdominal region as well. To slim your thighs, you'll want to focus on eating at a caloric deficit and losing weight. You won't be able to target the fat on your legs though, as spot reduction is a myth, you will just have to maintain a caloric deficit until your body decides its time to take the fat off of your thighs :)