Thinner legs; more muscular legs; shapelier legs - everyone has a different goal when it comes to their leg development and this must be reflected in the thigh program they choose to do.
First let's discuss a thigh program for those wanting thinner legs. Since your goal is to reduce the size you will want to avoid anything that is going to build up your muscles too much. This does not mean you should avoid weight training altogether, you must just go slightly easier in terms of the total number of reps and sets you perform. Aim to complete one leg workout per week doing about 15-20 sets per workout. Still use weights that are heavy enough that you are fatiguing within 10 reps to keep your strength level high. After that, be sure to include a lot of cardio training in your thigh program. Running is a great trimmer and if you are able to do this, probably one of the best choices.
Next comes a thigh program designed to add size. The number one factor when adding muscular size is providing enough calories to generate new tissue. If you aren't eating in a surplus of calories you aren't going to see more muscle. So first make sure you are eating more (the extra calories should be coming from healthy foods, of course) and then make sure when you hit the weights you are hitting them hard. You want to fatigue the muscles so that they break down and after given sufficient rest will grow back stronger and bigger. Aim to complete 2-3 leg sessions per week, ensuring you have at least one day of rest in between each one. Try and avoid doing too much cardio as well as this could hinder your gains. One or two sessions a week plus 10 minutes or so after a lifting session is all you really need to stay in cardiovascular shape and help promote good recovery. Keep them of moderate intensity though so as not to add too much additional stress to your leg muscles.
So whichever goal you have, make sure you train in a productive manner. Setting a guideline for yourself to follow before setting out is the smartest way to go about seeing results.