You should have muscle laying over these bones. You should feel a slight indentation at the end of the ribs. You should have a good sound loin that is broad and muscular. He's going to carry game, go under things, go up and down pitched banks. He's going to do a lot of running and that loin has to be strong and flexible.
Good condition equals good weight.
Conformation pose is an entirely different thing. The English Cocker should have a slight slope from the wither to the loin. Not an Irish Setter with a steep slope to the back and hocks standing far behind the dog. The English Cocker is moderate in structure and should be moderate in pose. They go together.