Your legs can be made stronger thus look bigger by a number of strength and resistance workouts techniques. These should be done every alternate day at the least or even with a 2-day gap, if the workout has been strenuous. Do remember to warm up prior to the workout and stretch for at least 10-15 seconds on each muscle group after your workout to avoid soreness. Remember to start slowly and build up over time so as not o hurt yourself.
You could start off your routine by doing squats and lunges, which are great, compound exercises that firm up the entire lower region of the body. To work and isolate the individual muscles, you can do leg extensions that focus on the quadriceps in the front of the thighs, leg curls for the hamstrings at the back of the thighs. For the calves, you could practice heel raises either standing or sitting. Remember again, you can do more repetitions with lighter weights (8-12 for 2-3 sets) and do fewer repetitions with more weights 6-8 reps and 2-3 sets.