It is appropriate that you have your Jersey Giant rooster standing next to a turkey in the picture, because Jersey Giants were originally bred as a meat bird and to be a substitute for a turkey. The information I read stated that they take about 18 months to fully mature. Your rooster may still be growing. 
Jersey Giant roosters are supposed to weight about 13 pounds. He is a big boy.

Jersey Giant roosters are supposed to weight about 13 pounds. He is a big boy.