When it comes to slimming down your legs, cardio can definitely be helpful. Cardio exercises help burn calories and can lead to reduced body fat overall, which may make your legs look slimmer. Additionally, cardio can tone leg muscles and improve their appearance. However, keep in mind that spot-reducing fat from specific areas of the body is not possible, so you'll need to focus on losing weight overall through a healthy diet and regular exercise routine that includes both cardio and strength training.