It's all about balance. I don't mean in your life, but in your squat. Fix where you apply pressure on your foot and you fix your squat. With this heightened stability, you'll become more adept in the squat pattern.
This will allow you to use much heavier weights at higher volumes while putting much less stress on your body. And, with better mechanics, the quads and glutes will contribute more, which means more muscle.
But finding and keeping mid-foot pressure feels awkward if you've been told to focus on keeping your heels down. You can be taught to think about this, but unless you're put into a position where you really feel it, you may never understand it.
There's an easy fix. Do a couple of warm-ups while standing and squatting on little plates, placing them not under your heels, but instead under the middle of your foot.
Thicker plates like the white 0.5-kilo plates are best, but if you're training at a big box gym you'll probably only have access to the thinner 2.5-pound plates. They'll still work.
Start by spacing the plates to fit your squat stance. Unrack the bar and step on them so that they're centered in the middle of your foot. Make sure that neither your toes nor your heels are flat on the floor. As you squat, apply pressure from your foot squarely on the plates and stay balanced.
This provides immediate feedback as to what mid-foot balance feels like. You'll know if your heels or toes press against the ground as soon as you start shifting to the front or back. From there, you'll recognize the feeling of mid-foot balance.
Doing a few sets with the empty barbell while standing on these plates, and maybe one or two with your lightest warm-up weights, will be enough to influence your balance for the better with all of your work sets.
You'll feel the quads and glutes being triggered more as you do these warm-up sets. Give it a try!