It depends on what kind of slim you're talking about. Lunges will tone them but might build muscle which will make them look thicker if you're little to begin with or have fat on top of them that you're not deling with. Running and swimming is aerobic, so it'll burn up muscle as well as fat, making u slimmer, if less strong than lunges. However it's important to realize that having more muscle through weight training and such will make your body burn more calories while at rest, so if you do it right it'll slim you down on your off hours as well as during the exercise. So there's pros and cons to both.
I have the opposite problem....good luck, it seems to be one of those body bits that defy all efforts at improvement...lol
you can try some aerobics exercise
Only way I ever found is less food and more excercise - these boots were made for walking ....
Raw vegan diet and aerobic exercise.