Do you sell your puppies with all confidence at eight weeks old as "Show Quality"? Do you then, when they don't reach the heights of your expectations, turn to the owner and say, "Whatever did you do to it?". The implication being that you could not have been mistaken and over-rated the puppy's prospects, but that it had been incorrectly reared by the owner.
Ours is a pretty mixed up breed when you see the different types of bodies on the dogs - but then the coat covers all that - so maybe you don't see them. This difference in bodies has been with us a long time. They were not standardized in China at the time the dogs were brought to the West, and each of the early imports had a different body. These differences, at least amongst the show dogs, are less marked now, but they may still carry the genes from their ancestors.
It is most interesting to note the varying modes of development in the different lines. To study this, each puppy of every litter needs to be fed on a controlled diet, even this can make a difference when possibly they eat different quantities at different ages.
It really can be you, the owner, not the breeder, who has ruined the puppy for show, but, breeder, don't be complacent - you can also be very wrong. You can only pick to the best of your ability or experience. Few, if any, dogs are genetically pure for only one type. Even those kennels who have created a 'strain' of their own cannot produce every puppy to their specification, though with experience one can recognise a certain type of body and have a pretty fair idea how it should develop.
Some lines alter considerably during their development, the stages of growth which take place may vary in their sequence from one line to another. There are those who put on a massive growth and weight early, and still end up well within the standard. Only your own records of weights for the particular line will give you the clue to the ultimate adult weight and size. Within any given line, of those I have reared, I would say the weight size ratio is fairly well regularized. Some I have reared have shot out in length of back between 6-8 weeks old, after which they have made steady and even growth to adult age. Others have grown in back length around six months of age, most disconcerting, as they usually end up too long; this is certainly a legacy of one of the old lines, photos of some English antecedents and Chinese paintings will bear this out. We don't want a short dog, but too long a back is inclined to roach and can have a weakness of spine. Noses have an annoying way of shooting at 4-6 months, however, they do not do this to oblige if the under-jaw happens to be too undershot! Alternatively the under-jaw may well go on growing after the upper, ending up with the dog showing its teeth. Some develop hind and forequarters as a horse, first one end then the other, unfortunately the front does not always get the message to grow, and the result - 'up behind' - only I call this type 'down in front'. Up behind has a perfectly normal front leg length, but is up at the rear, sometimes due to a too straight stifle. I have usually found leg length discernible at an early age, though I do recollect some of my early ones growing up on the leg until ten months old, however, I cannot remember how they looked at eight weeks. Those tails! They can be an enigma, perfect at eight weeks, loose when teething, flat, tight curled, half curled half flat to one side, when adult, and how important they are to the balance of the dog. "Ah, but the skull and head", you say, "you can at least be sure of that at birth". But can you? I grant that skull shape remains much the same, but some, large at birth, do not appear to grow! Ears and ear leathers, yes, they do not let you down. Eyes do not all develop as expected, some seem to get larger, others do not grow larger as the head grows, so appear to get smaller. It does not follow that a potential show specimen at eight weeks will finish well even when correctly reared, so breeder do not be over confident. Perhaps none of these things have happened to you, so you still say you can be sure of a show puppy at eight weeks, and it is still "Whatever have you done to it?". ! What may have been done to ruin it could be one of the following: over-exercised at the wrong time, under-exercised at the wrong time, run on the wrong surface all the time. Fed too much badly balanced diet, or fed too little and incorrect food. Over-supplementation of the diet, or insufficient supplementation when it was needed. In other words, bad husbandry. I have not even touched on a ruined temperament.
A dog is only as good as the genes it inherits, but it cannot reach its full potential if the environment is wrong, and, conversely, a dog well reared in the correct environment cannot finish a fine specimen unless born with the necessary genes.