this may seem obvious but think about it, men today need to go on specific diets and go to the gym every day and workout extensively for years to become body builders and have that muscular look, back then there were no gyms and food was limited and people were busy farming and working it's not like there were men who said we have free time we are going to spend hours working out, so how did ancient artists depict the muscular physique so accurately back then?
There's been a few questions that might help you. They're not about artists specifically but do describe how and why individuals might achieve those body types.
Did Socrates even lift? /u/ Iphikrates
How did people from Ancient Greece keep their bodies amazingly fit? /u/ Jaquestrap
Gym rat here. How ripped did ancient Greeks get? Iphikrates again.
https://www.reddit.com/r/AskHistorians/comments/46guc3/gym_rat_here_how_ripped_did_ancient_greeks_get/d051b11?utm_source=share&utm_medium=web2x&context=3This last one is another compilation of questions and not an answer itself.