The largest recognized, standard breed of chicken is the Jersey Giant with roosters typically weighing 13 lbs. (sometimes more) and hens 10 lbs. Cornish X Rocks can exceed the weights of Jersey Giants, but they are hybrids and not true breeds. The largest chicken ever bred was the White Sully, a hybrid developed by Grant Sullens from West Pont, CA. who continually crossed Rhode Island Whites with other breeds until he developed a super sized chicken. White Sully roosters could exceed 23 lbs. and were so ferocious that one male rooster named "Weirdo' killed two cats and crippled a dog. As far as I have been able to determine, the last White Sully died sometime in the 1990s.