My Marans aren't hatchery either so they are slower maturing but they do get big when fully mature. They are a good dual purpose breed and the hens lay well.My male I lost this winter was under a year old and he was massive for something not breed to be eaten. I needed both arms and determination to move him, though to be fair, I am a smaller human too. He also wasn't from a commercial hatchery, so he likely wasn't breed in the pressure to hatch as many eggs as possible
My current solo Marans cock bird has frost bite damage to his comb. He lost it over this past winter. He was about 18 months old at the time of this photo. He has filled out a lot more since this photo. He is now 2 years old.
This is his sire and dam.