Legs are a pain in the you know what to train. This is the reason why people neglect them completely or oddly enough fall in love with training them. You have to approach leg training with a reckless abandonment, especially if you want them to grow.
When it comes to training quads (the fleshy mass composed of 4 muscles on the front of the thigh), there are some exercises that seem to be the most popular. They are squats, leg presses, and leg extensions. Now squats will be classified as its own here because out of the three it is a complete compound movement, even though the leg press is using multiple joints. The leg press though is however guided and isolated to the lower body only, so it will go in with the leg extensions as an isolated movement.
So, when it comes to building quads, which of these two groups seem to be the most effective; compound movement (squat) or isolated movements (leg press and leg extensions)?
A compound movement is any movement that involves multiple joints and muscles, so this in return will train more muscles at one time, however, will not isolate a particular muscle as well. So instead of 100% of the isolation going toward the quad from a leg extension movement or leg press, maybe only 40-50% go into the quad from the squat, but the upside as stated with the squat is other muscles are being hit as well such as hamstrings and glutes, working on overall development of the lower body.
It also should be noted that squats can be altered to become more isolated to the quads as well. A front squat takes a lot of the engagement from the posterior chain (glutes, hamstrings, calves) and puts it on the quads. So the front squat, while still being a compound movement, becomes more of a hybrid or enhanced isolated movement as well, all because the bar went from being behind the back to the front of the shoulders.
So, when wanting to develop a particular area because you are looking for more balance or just size, isolated movements are the best. However, compound movements are still amazing in their own right for overall development of a large section of the body, not just a particular part. But compound movements can be altered and adjusted to mimic that more of an isolated movement such as changing a back squat into a front squat for more isolation on the quads.