zerchers shouldnt be looked at for quad size and strength in the first place. Its a wide based squat that mimics the mechanics of a PL squat. its heavy on the posterior chain and brings the upper body and core into the movement. generally speaking, its not a struggle to hold up the bar on a front squat if your form is right. its always a challenge to hold the bar with a zercher which is where the challenge to the body lies. of course your legs shouldnt give out, but the logic is the same as someone who leg presses 800lbs...should they be able to squat 800 too? but their legs dont get tired....
the point is, to challenge that movement to the point where SOMETHING gets tired and breaks down. I regularly do these up to 2-3RM and where my legs dont get tired, I may discover that my glutes are lacking or my hams start to buckle, or my erectors are wrecked....somewhere there is a point to doing the exercise that will help.
its the movement itself and the emphasis on weakpoints that makes the difference in why the weight is lower. put the body in a weak position and force it to adapt. when you get strong at these, i guarantee...the carryover functionally is great!