It's the same as any other body part, you have to get the muscles you want to target to move the weight. You can squat 400 for reps, but it doesn't mean that your legs will get huge (though they won't be small either). Look at a lot of the lighter weight Olympic lifting guys. They have big legs, but nothing like a bodybuilder and the Oly guys squat with much more weight and way more volume.
You might want to give front squats a go for a little while too. These target the quads a lot more directly than back squats. Full ass to grass reps with these will blow your legs up.
Another thing is you might want to try lowering the weight for a few sessions. Sometimes when you are doing crazy poundages you can't really concentrate on squeezing the muscles you're trying to hit. This is especially true for leg press. I always laugh when I see guys with no legs throw eight plates a side on the leg press, and do retarded reps. This accomplishes nothing except ego stroking. Take it to the stops with four plates a side for twelve solid reps while squeezing your quads through the whole movement, and see how you feel afterwards.