I think Jersey Giants don't actually make very good meat birds (that is, raised strictly for meat, to be slaughtered at a young age) because they take so long to develop to size. If you want to raise birds strictly for meat, you'd be better off going with Cornish or Cornish Rock crosses. Those I believe are slaughtered commercially at around 6 or 8 weeks old.
Orpingtons, Barred Rocks and so forth are considered dual purpose birds because they lay well, and after their "useful" life as layers, they are large enough to still have good table qualities, whereas birds like Leghorns that are strictly layers don't really have any meat to them at all (although they may be good for soup).