growth limit on muscle
Natural athletes have to deal with MSTN gene.


  The MSTN gene provides instructions for making a protein called myostatin.
  
  This protein normally restrains muscle growth, ensuring that muscles do not grow too large.


Just look at the top natural athletes. There are a lot of them who train for 20+ years and still can't be as large as those athletes who take steroids. Here are some examples of professional natural athletes(Bobby Pandour, Georg Hackenschmidt)



With steroids you can exceed all natural limits up until your body meets its boundaries, often accompanied by serious health issues.