Dogs who are altered young tend to grow larger then their siblings who are not altered. Harmoens directly affect the growth plates of the bone and removal of them at a young age causes a delay in the closing of the growth plates.
Howeve, they often do not develop the bone mass and the muscule mass of dogs alterd after maturment.
In a breed that weight pulls, their density is a factor to their ability to work, so a leggier dog can not do the work. Muscle mass varries animal to animal of course, but for the most part correct, although if well excercised the diffrence is not hugely obvious.
I had my rottweiler neutered at the age of 1. I heard different things about their growth if neutered before the age of 1. I was told he could be smaller than average, etc. He is now almost 5 years old and weighs 90 pounds.
I had my boxer spayed at 6 months. She is 57 pounds but is small in size compared to most boxers I have seen.