I just learned this from a 2010 book titled, "Designer Genes".
I had always thought it was a combination of body type and testosterone that limited muscle development. I'm sure that may be part of the explanation but now there's Myostatin to think about.
Myostatin is a hormone that regulates muscle size. The greater the muscle mass you achieve, the more of this hormone you will produce. Its job is to inhibit any further muscle growth. So this explains why you may have to work harder and harder to add just a little extra muscle. You may want to be more and more muscular but your body (hormones) is putting on the brakes. Thanks to Myostatin!
Sometimes there's a genetic abnormality whereby no myostatin is produced. In this book there's a picture of two doberman dogs, one is normal and very slender, the other did not produce any Myostatin and is all bulked up with huge muscles. It seemed to be about twice the size of the normal dog. And it didn't have any gym membership, at least as far as I know.
Our body type is determined by our genes. That's why we look a lot like our parents. We are tall or short, small boned or big boned. And I assume the same applies to our hormonal system. So some people can develop a lot of muscle and some cannot. It's as simple as that.
If your hormonal system (i.e., Myostatin etc.) is putting the brakes on muscle development, no amount of protein shakes, eggs or meat will help.
As far as working out, we are up against the "law of diminishing returns". You can double your work-out-time in the gym but you may only get very little extra muscle, if any. Thanks to Myostatin.
In light of the above information, what would you advise others about diet and exercise? I would recommend a regular diet with perhaps a little extra protein and limit workouts to no more than one hour per day. There's no magic protein formula that's going to make you bigger. It's all advertising hype, in my opinion.