Genetics will play the biggest role in determining the build of your dog, followed by both diet and exercise. At 9 months there is no reason to worry, she is still a growing pup and dogs mature at different rates. It's unfair to compare the build of a 9-month old pup to that of a fully mature adult dog.
"No! If a man does not soon pass beyond the thought: "By what shall this dog profit me?" into the large state of simple gladness to be with dog, he shall never know the very essence of the companionship which depends not on the points of dog, but on some strange and subtle mingling of mute spirits." -John Galsworthy