I wouldn't worry bout it too much. The setter could have been a bench setter which usually run around 55+ pounds but are the size of an irsh setter or grey hound. Your pup could also have been the smaller one of the litter perhaps that's why he's whining when he eats.

Doesyour puppy have dominante personality? Turn him on his back and see what he does. Does he fight or submit? This will tell you how easy his training will be and will answer some of your questions. Another thing, socialize him and if he growls at people and kids then you may want to take him to a dog behaviorist (sp?).