Greeks were all about creating the perfect human being. Their statues were buff people or thin and curvy women who looked beautiful and almost unreal. They believed that there was an ideal of human physical beauty and that it should be shown in art and that all people should strive to look like that which is why sport was so important then.
Romans on the other hand wanted to be more realistic. They didn't want to show unreal statues of people but wanted to strive towards realistic depictions. This is why you will often find statues of people who are small, obese, bald, have a hunch, or similar things. They wanted to depict humans as they looked.