I would suppose it would be because Greeks married young. Very young. Young Greek women in particular lived very restricted lives - they were not allowed to associate with men outside their own families. I think their lives resemble women who live in modern day Saudi Arabia. I'll provide a link to what their lives were like. But in regards to the statues themselves you are thinking of adults - I think they were designed to resemble people around the age of 14. If you think of them like that the size of their, uh, body parts would be small - you're talking about people who are basically children.
Here's the link;
http://www.ancient.eu/article/927/