I disagree. Genetics have very little influence over one's physique as long as other more important factors are considered, such as the person's activity level, amount of sleep, eating patterns, and work out routine. For a given person, those will be the differentials.

Also, focusing on genetics ignores the fact that the exercise routines would be different for each person. Without knowing one's own genome sequence, a person who tries to stay active will learn what their body's preference is in terms of exercise. For me, I focused on muscle size. Other athletes focus on weight. Others focus on speed and conditioning. One will figure out which sports their body is meant for soon enough if they exercise regularly. The factors I've mentioned are also ones the person will learn to adjust--not their genetics.

I don't know much about my genetics except that I'm taller than my parents. But I have observed changes in my body after working out regularly. These changes included improvements in metabolism, hunger, body mass, mood, confidence, speed, and posture. My genetics presumably did not change in the same span of time. (Neither are they something I can control.)

So therefore, the person's activity level, amount of sleep, eating patterns, and a stable, consistent, work out routine are what matter the most.

Comparing two different people entirely (in other words, two people with different genetics) is purely theoretical and not practical, as each individual would have had different prior sports experience, confidence level, access to quality foods, knowledge about nutrition, free time, and more. Even if one did somehow match two people with the same activity levels at a given point in time, they are almost guaranteed to have entirely different results in the future for those reasons.

Similarly, two twins with identical genes will have huge differences in fitness if one considers the factors I've mentioned and one doesn't. Also, their metabolism levels, "bicep size", and other body features will be noticeably different.

Genes might cap one's maximum ability: some people have genetic diseases affecting their physical ability, while others have "caps" that are greater than those of other people. But while in the first case it might be easier to zero in on somebody's (physical) limits correctly, in everybody else, it is impossible to tell how strong, fast, athletic, or muscular a given person can be, or what their metabolism will be like, when they're athletic. Progressing from spending 20 minutes exercising, three times a week, to playing sports for 2 hours at a time three times a week, lifting weights for 90 minutes four times a week, and running for about 30 minutes a few times a week, all at once, does not depend on one's genes. Also, I want to point out that it may only be record-setting Olympic athletes who truly get close to their genetic caps, and therefore genetics are not a factor for most normal people.

Finally, regarding bicep size, here is a quote from the first search result on Google:

"Genetics play perhaps the most important part of not only how large your arms can be but also their shape. Now if you are not a genetically gifted bodybuilder do not take this statement as a cause for you to have a yard sale and sell your weights. True, you may not be able to develop a 21” arm but this will not prevent you from sporting an 18” or even 19” well shaped and defined arm and whats wrong with that? A well shaped defined 18” arm is much more impressive than a flat chunk of flesh that measures 21” anyway. It takes a lot of time and experience before one can say for certain any lack of arm development is due to genetics. Dont be quick in jumping to this assessment; its the lazy way out."

http://www.criticalbench.com/armmass.htm