Genetics and nutrition will help determine the size a pup will grow to. There is a fad these days for overly high protein grain free kibbles and those are not so good to feed any dog or pup. For a mid size dog like a pit bull I like to feed a quality kibble with from 25% to 30% protein, not higher. I also supplement with raw meats. I like to see meats in the first 5 ingredients and no peas or having it way down the list. It's been shown that grain free foods with high pea content causes heart problems in dogs. The FROMM Puppy Gold is a good food, 27% protein and 18% fat as I remember, plus it has no wheat or corn in it, and no peas. They have a Gold line for adult dogs too.

And you can feed a quality adult food to pups too. As said above with the giant breeds like wolfhounds you do not want them to grow too fast so an adult food is best. A good worming program is helpful too. As to size variation in a litter, my current gyp is 40 pounds, she's from an 11 pup litter and size as adults has ranged from 75 pounds to 40 pounds, with most being about 50 pounds, parents were 60 pounds and 50 pounds, it was a loose line breeding.