How Much Weight to Use on a Leg Press for Thinner Thighs?. Part of the series: A Stronger Workout. If you want to get thinner thighs while doing leg presses, you're going to need to pay attention to the amount of reps you do as well as the weight. Find out how much weight to use on a leg press for thinner thighs with help from an experienced fitness professional in this free video clip.