Testosterone tells your body to grow muscle and be stronger. That is why men look different than women, and women who take testosterone and do not train get muscles like men.
Males given various doses of testosterone, with no training, get bigger muscles the higher the dose.
The studies go up to 600. The result stays the same up to the highest dose. More testosterone, more muscle.
It might still work at 700, 800, 1000. 1200 - but we do not know, because the studies do not go that high. At some point we know anecdotally that the dose stops being more effective. I don't know where that dose is for me, because I have not reached it, but I have heard IFBB pros (more than one, perhaps half a dozen) claiming that when they run it higher than somewhere in the range of 1250-1500 there was a point of seriously diminishing returns.
Now combine that knowledge with intense weight training and a diet planned for muscle growth, and you get results.
Yes, 300 grows more muscle than natty, too, even without weight training. But the effect is better at 600. Keep in mind those studies are on healthy males who are not weight training.