The local Benelli rep bought a pup from a really large line of Labradors. He wanted a really large Lab, so he over fed the pup, and used a food with too much protein and calcium in it. The pups growth was so abnormal that he got to where he couldn't stand up. The pup ended up living in a crate, on a special diet, and not allowed any exercise for over 6 months while the rest of his body caught up with the long bones in is legs.

I haven't spoken to Mike in a while, so I don't know how the dog is doing now.
I did a lot of research on the proper diet for Jasmine prior to picking her up. One item that came up a lot was feeding the proper balance of calcium and protein. I takes a little longer for the pup to reach their full growth, but when they get there all their bones and ligaments are in proportion.
Like Run said, there are a lot of quality dog foods out there, and many of them are specifically formulated for large breed puppies. They tend to cost a little more, but for the first 9 months to a year it's well worth the expense to insure your buddy has a long and healthy life.