The history of the AUSTRALIAN SHEPHERD was developed in the UNITED STATES from a LITTLE BLUE DOG - about 12-14" , thought to be brought over by sheepherders from Australia. Hence the name Australian shepherd. But likely was a dog from the BASQUE region of France. All the historical recoreds talk about the LITTLE blue dog. Size word LITTLE. not the big dog that the Standard Australian Shepherd is. THE LITTLE BLUE DOG was bred to every breed of dog that was herding type. The group breeding for larger dogs to herd cattle got to AKC with a breed "standard" first. with dog being over 18". So people assume this is the "original Australian Shepherd" and the other sizes were bred down from them. Was actually the REVERSE. THE AUSTRALIAN SHEPHERD, standard, mini or toy has only been around since the early 1930s, less than 100yrs.
After having a Standard Aussie and breeding the MINIATURE & TOY AUSSIE, it is my opinion that the variation in the STANDARD, MINI, and TOY developed due to the other breeds of dogs that different breeders used in their programs. STANDARD AUSTRALIAN SHEPHERD breeders bred the LITTLE BLUE DOG to LARGE BREEDS, COLLIES, Bernese/Swiss Mountain dogs, with the BIGGER IS BETTER attitude. Standard Australian shepherds are 18" and over at shoulder and 40+lbs. I think miniature breeders used medium sized breeds, border collies, shelties, etc - Miniature Australian Shepherds are from 14"-18" 20 to 40lbs. TOY breeders used toy breeds to go under the 14" size. . But the original "Little blue dog" intelligence, loyalty and looks was the basis of the breed.
Standard breeders can get a TOY once in a while from a STANDARD TO STANDARD breeding, because of the genes of the LITTLE BLUE DOG. I look at standard Aussies and see the looks and traits of COLLIE, Bernese Mt Dog, and have seen an "Aussie" that looked like a merle St Bernard. I look at minis and see looks and traits of the border collie, shelties, even corgi. I do not knowingly use a dog from non-registered "mini/toy" Aussies in my Aussie program. It is how the breeder selectively breeds the dogs they have based on the traits of the breeding dogs and their resulting offspring, which determines how the line/breed develops.
All breeds of dogs are initially a crossbreed of other breeds, then selectively bred for specific traits or traits changed based on what the "breeders" or public wants. The Pomeranian was once a 40lb SLED dog, is now a 10" house dog?? and merle has suddenly "mutated" into many breeds.
The Mini Aussie and Toy Aussie have not been around very long in the scheme of things, and the original registry was an "open" registry, meaning that if it looked enough like an "Aussie," then it could be registered and used for breeding with no knowledge of its parentage. But some pedigrees have been lost for various reasons. But most Toy & Miniature Australian shepherd breeders Register with American Stock Dog Registy, ASDR, and have generations of verified pedigree behind their dogs. If dogs are registered "CKC" or APRI - the were not either purchased with breeding rights - ie don't know parentage/pedigree.
Many registries started as "open" with restrictions for registration coming about LATER and the books being "closed" unless there XX generations of verifiable pedigree.
Each size group also incorporated the traits and genes, good and bad, from these other breeds utilized. Most of the health problems associated with Aussies are from these other breeds. Most of the eye problems are COLLIE ABNORMALITIES!. The Toy is the less herding of the three sizes because of the other breeds used. And so far, has much fewer health problems than the other sizes because of the breeds used, and selection. The original dog from "Australia" was a LITTLE BLUE DOG (little being the size word). But the Standard Australian Shepherd group made it to AKC first. So people assume that toy and mini Aussies are bred down from Standard Australian shepherd and is technically the reverse. To confuse matters more, a group of Miniature Australian Shepherd breeders wanted to go AKC, and had to use name Miniature AMERICAN Shepherd to register Minis with AKC. They are considered a different breed - as the gene pool is limited and eventually those breeders are working towards something "different" and will be a different breed through selection.