I think that they do take a while to achieve their full weight.Mine were quite large by 6 months, but now at about 20 months are about the same size, but much heavier.
Mine don't eat large amounts of the food my other chickens eat, nor do they spend all day hanging round the back door and following me everywhere after more food. They forage over several fields and eat lots of grass etc. Very good at self-sufficiency!
I think this breed is under appreciated....they have lovely black feathers and lay very well during the longer days. They are said to have some of the best-tasting meat of the chicken world, but not being a meat eater, I cannot vouch for this!
Edited by pgpoultry - 11/20/12 at 8:27pm