Some call an Imperial a runt or a dwarf. I believe the first tiny types could have a better chance at being a runt. Imperials are just to small to be the runt, they are a work of years of Selective breeding, and I believe started centuries ago . Look at your Poodle sizes, Giant Standard, Standard , Miniature, Toy and Teacup. Do you belive the Teacup is a runt produced by the Giant Standard, that is ridiculous.
Size is the result of selective breeding.
Imperials are created from selective breeding of Standard Shih Tzu females and mating with a smaller male. Your results will always be good if you use genetically sound parents. Breeding Imperial females in my opinion is wrong because she can go c-section or they die in Whelp. Using a Imperial male that is sound is best to get your Imperials. If you find that your Tiny types and Imperial puppies are not healthy than it is because the parents you used were not sound .