The reason for the size of the bodybuilders back in the day was as much because of how strong they were as it was because of mental focus and muscle isolation.
Arnold could rep 405+ on the bench and at one point I believe deadlift close to 700lbs.
Franco could deadlift, squat and bench a fuck ton of weight.
Even the small, ripped guy Frank Zane in his prime could rep 315 strictly on bench if I remember right. And if you look at pics of him before he got ultra shredded before a comp you would see he was actually quite yoked.
There are strong bodybuilders nowadays. But, there are also weak bodybuilders.
This reminds me of the old concept of the weak, pumped up guys. Jay Cutler would be in this category. You can use relatively lighter weights and do higher reps and volume and generate massive muscle pumps and if you do it 2-3x a week your muscle will get bigger. This is what I did for forearms one summer and I shit you not after about 2 months of this type of training my forearms were almost as big as my upper arms. The funny thing was is that even though they grew hugely, they only increased in strength about 10lbs.
This is a very old concept and it does work. It's just not used by many people because it is almost completely aesthetic in nature. That and the muscle burn and pump you get from doing it are pretty unpleasant.