He is taking about putting on mass and your suggesting 20 reps..

Maybe if he wants to look like a swimmer. Most Huge bbs I have seen and talked to keep it between 4-6 or 6-8, and always fail on the last one.
Squats develop the whole body PERIOD, they are the cornerstone.
In most gyms people don’t work out there legs as much or as intense as there upper bodies, hence the uneven look.
Compound movements will put on the most mass because you are using your secondary muscles to help the muscle you are working out to be more intense, more breakdown, more rebuild. EX: A stand up ez bar curl or strait bar curl will put on more mass, more evenly than doing preacher curls.