Want more muscle? Trigger protein synthesis often. Here's how.
Natural? Train More Often
I normally recommend a higher training frequency, both in the form of more weekly workouts as well as hitting each muscle (directly or indirectly) frequently during the week. That's best for both enhanced and natural lifters, but it's even more important for naturals.
While enhanced lifters can still get significant gains from hitting each muscle once per week, naturals need to stimulate each muscle at least twice a week, and ideally three or even four to get significant growth past the beginner stage.
The key to building muscle is triggering protein synthesis, the process in which your body uses amino acids to build tissue. If you want more muscle growth, then protein synthesis has to be elevated to a higher degree and has to stay elevated longer.
After a workout, protein synthesis remains elevated in the trained muscles for around 24 hours – slightly more or less depending on the workout. By training each muscle only once a week, you aren't keeping protein synthesis elevated for long in each muscle. Not only that, if you're training each muscle only once a week, you might actually lose the positive adaptation (muscle growth) by not stimulating it soon enough.
This is called "involution." The body doesn't want to carry extra muscle mass that's not useful. So by waiting too long before hitting a muscle hard again you might slowly lose some of your gains. Not all of them, but it can certainly diminish your rate of progress.
A natural lifter needs the training sessions since they're the only significant protein synthesis trigger he has. Eating also increases protein synthesis, but in a systemic pattern and to a much lesser degree. So the natural lifter is far more dependent on each workout to stimulate growth.
Steroid Users and Protein Synthesis
On the other hand, enhanced lifters are artificially increasing protein synthesis with drugs. The steroids keep protein synthesis elevated around the clock. Their workouts of course increase protein synthesis to an even higher degree, but hitting each muscle more often isn't needed because the overall rate of synthesis will stay elevated all the time.
Some studies have found that taking anabolic steroids led to some muscle growth without any training. I'm not saying that people get huge that way, but it certainly prevents losing the adaptations that can occur from only training a muscle once per week.
Natural lifters need the workout to elevate protein synthesis. And since it only stays elevated for 24-36 hours after a workout, they need to hit each muscle more often if they hope to make significant gains. Enhanced lifters will also make more gains with higher frequency training, but it's not as necessary for them.