It feels like the only way to achieve the bodies depicted would be to actively train for a very fit body.
Now I know they lived a much different lifestyle back then- one that was much less sedentary, and I also recognize that depicting men in such a glorious and ideal way is obvious for a statue sculptor trying to make an epic piece of art, but it still feels like the men are in such perfect form, they would literally have to train all areas of their body like a bodybuilder, so to have a reference point for what muscles would look like once thoroughly developed there must have been some people the sculptors used.
Obviously if there were 10 out of 100 men in a social group who were in very good shape then they would stand out.
You have to think about why these statues are being made, usually its for one of two reasons. One is that someone has paid to have a statue made of them, another is that the sculptor is honoring a person or a god. If marble and bronze were cheap and easy to work with we would likely see many varying styles of statues but the opposite is true, thus if a sculptor has a chance to make a work he's going to do the best job he can. These statues represent the Greek idea of 'perfection'. In the linked photo we see Apollo, but the model for the body is was what an ancient Greek sculptor would think to be perfect, the body of an Olympian athlete. Thus, the answer is in your prompt! These younger men, who exercised and trained in the nude in the gymnasium for hours almost every day, made the perfect models for statues. And the sculptors would try and create a statue that would be seen as perfect. So often we see a statue with the head of 50 year old statesman but the body belongs to a 20 year old athlete. Perhaps the most popular example of a perfect statue is called the Doryphoros. This statue had perfect proportions and it conveys a body in motion. Paul Zanker in the Power of Images in the Age of Augustus, points out The Doryphoros was so good that when the first emperor of Rome had statues made of him he copied the Doryphoros. So here we have someone paying for a statue who wanted to be seen as perfect, and what sculptor would disobey the emperor? These guys were not fat or unfit but they did not have the bodies of young Olympians. Another note that applies to the Romans it that if you look at Augustus in the photo, you see he has a youthful face, but when that statue was made he was in his later years. The style used for his face is oddly called Realism but before Augustus another style was popular which was called, Varism. With Varism we see the imperfections of age, and that was to show the man as older wiser person worthy of respect. So these statues are purposely trying to show their subject as perfect but the body of the statue is very often not representative of the subject.