The proportions of muscle fiber types (mostly genetically determined) a person has has a big impact on their appearance and propensity to do well in various sports (assuming the person is actually fit and active not a sedentary couch potato). One of the most common examples is the difference in physiques between elite marathon runners and sprinters and the relative proportions of fast vs slow twitch fibers found in these athletes.
Since fast twitch fibers are larger and able to contract more powerfully, a person with a very large percentage of them will naturally be better at sports requiring that kind of performance, and their muscles will generally get bigger than a predominantly slow-twitch kind of person under the same training program. The training involved in explosive sports also tends to cause the muscles to get larger and stronger.
So, there are several layers contributing to the general appearance of elite sprinters: first you have the individuals with muscle types that typically respond in a more visually obvious way to resistance training being selected or drawn to the sport. Then you have them do a lot of training in general which will get their muscles developed and body fat minimized, and then you add in doing some types of training that specifically favour strength development with a side effect being an increase in size as well.
It’s interesting to imagine taking a current Olympic sprinter and marathon runner, and have them switch sports and train and compete only the opposite sport for 10 years… No doubt their muscle composition would change somewhat, but I think the sprinter would still look rather muscular and more developed than his fellow marathoners (who’d likely always outrun him). I also think that while the former long distance runner will get a bit bigger and more developed, he’ll still remain thin and wirey overall and probably never get as fast as the other natural sprinters.
So, I think it’s fair to say that sprinters look the way they do because they start with a genetic muscle composition that gets bigger/stronger easier than for the typical person, then they spend a lot of years doing very intense training specifically for powerful movements which only enhances what they started with. Diet and a healthy lifestyle required to maintain peak performance also keeps their body fat low which also enhances their muscular look.
Of course there are a small number of individuals who would prove to be exceptions, but I think in general the above is true.