Unfortunately if your Jersey Giant is from a hatchery, it will likely stay small. Even if it came from a breeder, unless that person is selectively breeding, your bird may never get very big.
If you want Jersey Giants that are really giants, you need to seek out a breeder who selects to the Standard. As others have said, hatchery giants aren't big. Attached is a pic comparing 2 hens. The larger hen is 7 months old in the photo, the smaller is 2 years. Both are from breeders, one just...