Hi, as per Templelands reply this sounds like a blister on the breast bone and is harmless.Hubbard's and other large meat birds are prone to this it comes from resting there immense weight all on this point on the keel of the breast.
For this reason Hubbard's shouldn't have any perches or anything resembling a perch that they can rest on such as a door frame to a hut.
I keep a thick layer of shavings on the flaw of my ones hut,and I just sprinkle a fresh layer over the top every day,this has stopped this problem.
Graham.