My Brahmas are very eager to breed asap. Only certain ones are kept for a gene expression study we are a part of. Last year 2 roosters were kept out of the flock and never got to breed (poor fellas). When we processed them, they went into the freezer weighing 6.5 and 7lbs at 10mos old. The 2 roosters allowed to breed were eventually butchered and they weighed in at 6.5 and 7.5 lbs at 12 mos old. Each pair were brothers but from different fathers and were fed the same feed. The non-breeders were about 30" tall while the breeders were only 28" tall.
Genetics determine what they can be. Good nutrition makes sure they get there.