Working out and stretching are absolutely essential.  If you don’t stretch, you end up like Mr. Meathead who can’t actually touch the small of his back because his muscles don’t stretch that far.  You may not end up that extreme.  Btu you get the basic idea, nobody really wants to reach that point.  But some of the most flexible people I’ve seen have been really big, muscular guys who have taken the time and care to balance things out.  Yet they have retained larger muscle mass. 

 That being said, the reality is that while some people follow pilates and yoga to be slender and lean, it doesn’t exactly work that way.  Your body’s bulk and shape at basic level is actually determined more by your genes in general, especially in your calves.  This is not to say that you shouldn’t stretch, you should definitely stretch.  But unfortunately, stretching does not create longer, leaner muscles or a more slender body in general. 

You will become more flexible, better able to do certain things, and so on, but your body type will not change.  There are two types of muscle fibers, and on average people have a 50-50 split of fast twitch and slow twitch muscles.  Higher grade athletes have reached those levels because they were born with the right muscle fibers along with other attributes such as the fact that one famous swimmer we all know is basically built like a fish.  But if you have more fast twitch fibers, you are more likely to bulk up.  That is the simple fact of things.