
This is the amount of formal exercise suggested not the total of all exercise, puppy initiated exercise does not count it is the amount of time lead walking that should be kept to the suggested amount that you have read about.
Personally I thing the main difference between large and small breeds is that you have to be more careful with large breeds and continue this care until the dog is older. If your dog is a BT then by the time he is 9 months restriction on exercise is probably not needed as long as common sense is applied no dog should be totally exhausted by it's enforced formal exercise. If on the other hand it is a Border Collie you would need to continue the care until the puppy is fully grown.
Think perhaps that is the rule of thumb, care needs to be taken until the dog is fully grown and that one should never on a regular basis walk a dog till it is exhausted.