I have a couple of friends, one of whom is a gymnast and the other is a triathelete. My gymnast buddy trains 6 times per week and while by bodybuilding standards would not be considered huge, is very lean and muscular. He looks better than most people who you would see at a gym lifting weights. He weighs 155 pounds and I am estimating his bf somewhere around 5-7%. As far as strength, he can do pushups and chinups (including one arm chinups) all day. The last time he was over at my house I loaded my barbell with 300 pounds (all the weight I had) and he had no problem deadlifting that weight - double his body weight. In his case it is certainly not genetics; his mother is 5' and maybe 100 lbs; his father is skinny like a stick and has a beer gut. His older brother has a body like Woody Allen.
My triathelete friend trains 5-6 times per week - she runs, swims, and cycles. She is lean, toned and has very muscular calves. Again, very strong and in excellent physical condition. I have never met her family so I can't speak to the topic of genetics.
Here are some observations that both have in common:
neither has ever lifted a weight; they don't train 3x per week doing 3 sets of 10;
pound for pound they are two of the strongest, best conditioned athletes I know;
neither of them pay much attention to macronutrient intake, calories, etc. From speaking with them they eat nothing special. They eat 3 balanced meals including meat, veggies, salads, etc. They snack on things like nuts, fruit, carrots, etc. I rarely see them eat junk food and have never seen them buy supplements such as protein powder or creatine.
their bodies look better than most of the people I see at the gym and I am willing to bet pound for pound they are stronger too.
I am going to follow up with them and do a detailed analysis of their training methods, along with a few bodybuilding friends and compart the different philosophies and techniques. I will let you know my findings.