One of the critical points regarding the squat is to make sure your knees stay on plane. Your knees should not cave in or out. This will cause a torque force to be placed on the joint. A force that the knee was not designed to withstand. Failing to address this will result in a several problems, the first being, reduction of performance. If you fail to address this, you will be limiting your potential. Secondly, would be an increased injury risk.
Fixing these issue isn't hard. But it does take a lot of mental tenacity, and some time. You are going to have focus, on every rep, pushing to keep your knees out, over your feet. Especially as you are squatting deep, or off a box. The key spot to watch, is your femur, or thigh. It should be inline with your foot. When you look down, your thigh and foot should be running parallel. If your feet point straight forward, then your thigh must run straight forward. If you feet point out, then your thigh must point out. keep your knee over your foot.
Reduce your ROM, decrease weight if you have to, spend the time getting your form correct. It wont take too long, and then the weights will start to shoot up, and you'll see some new size in your vastus lateralis, outer thigh. That you probably have been looking for.
If you want to keep trying the box squats, go for it. Just make sure you are not letting your knees cave in. If they continue to give you problems. You should then drop the box squats, and switch to front squats or back, front preferred since it's more quad oriented. Work on keeping your knees true on these exercises and try and get deep. You will probably find that as you get deeper or up the weight, your knees will start to cave in again, so your going to need to find the "sweet spot" the point where as you drop down, your form just starts to fall apart. This marks the end range of motion. Keep looking for more depth, but don't sacrifice your form.