I'm a pure squatter and always have had good quads. That said, I'm not a bodybuilder. If I was, from what I know, to build giants quads without squatting you'd do smith machine squats, hack squats, leg presses. Sissy squats were always purported to hit the outer sweep, but I never did them, because you don't use weight. I never cared for hacks, seems the machines back then weren't made for all sizes, I'm pretty tall and always had knee pain. Then I would put my feet so far up on the platform that my lower back would rise at the bottom. Smith machine squats lock you in, don't give you a natural feel. Leg presses are probably the best option. Put you feet about mid platform and narrow. If you go to high you hit hams and glutes. Too low your knees will hurt. There used to be this belief that angling your toes one way or another would develope different areas but I don't believe it.