You need to develop the overall mass of your thighs to bring them upto where they are proportionate with your upper body. Great size only comes about by lifting heavy weight, especially with exercises like Squats and Leg Presses. But modern bodybuilders need to develop legs that show as much quality as quantity. Clear and evident definition in the thigh area, with striations and cross-striations standing out as if revealed in an anatomy chart. The basic mass-building exercise for quadriceps is Squats.
Squats have a complicated mechanical effect on the body.
However, Squats may be more or less effective depending on an individual's particular proportions. Sometimes exercises like Front Squats are also needed to more directly work the quadriceps and lessen the involvement of the lower back muscles. Heavy Leg Presses also contribute to building massive thighs. Leg Extensions, which isolate the quadriceps themselves, are more of a shaping and defining movement. Good muscle separation and definition obviously cannot be achieved unless you diet to severely reduce your body fat.