It is unethical and unsafe breeding practices that produce very tiny dogs and we cannot even envision such a day as when this would be accepted by any reputable kennel club, and certainly not by the AKC.
Those particular breeders will take 2 dogs of smaller than average size (known as runts) and pair them together. There are no special "royal" bloodlines. The resulting litters are not special. They are puppies that are more prone to health issues such as luxating patella, hip dysplasia
and collapsed trachea
to name just a few.
Now, let's talk about Shih Tzu size that is larger than the standard.
This is to be expected with any dog breed. The standard is a guideline. It is a guideline to follow as to what your goals will be. Therefore, there will always be Tzu who are bigger in size that one may expect.
There are actually 2 different reasons for this:
1) The dog may be overweight - thus his true size should not be larger than normal - changes can be made to bring the dog to a health weight.
2)The dog's may have a larger body frame and bone structure than expected - size will remain. Once a dog is accepted into a family and is loved, size should not matter and the dog, we assume, would be lovingly accepted no matter what
Now, if a Tzu is overweight, a plan of healthier eating along with exercise, over time, will bring the Tzu back to a proper weight. Carrying extra pounds will put stress on the heart. It will also put stress just about everywhere on the body, particularly the knees, hips and back. The best method to help with this issue is to never feed without planning. The dog should be on a schedule of 1 to 2 meals per day depending on his age. Snacks should be limited and time should be taken to add fresh vegetables such as baby peas and baby carrots to meals. Also, fruit can be added with blueberries, raspberries and mango just some of the healthy fruits that a Shih Tzu can eat.
Exercise should be increased slowly. If an owner normally takes their Shih Tzu for a 20 minute walk once per day, this should be increased to 30 minutes.
The key is to increase the length of time and not the pace (unless your dog saunters along so slowly that it barely counts as exercise....in that case, the pace should be a bit more brisk). In addition, some extra cardio such as taking 20 minutes, 3 times a week to allow your Shih Tzu to run around in a safe, enclosed area... Or taking that time to play an active game of fetch will be terrific.
Do not expect a fast change. The healthy and proper way to lower body fat and increase lean muscle is to plan on this happening gradually. Once the dog has lost the extra pounds, one must be careful to not fall back into bad habits (giving table scraps, etc) and to continue with a good feeding and exercise plan.