Well teacups are many, many generations in to breeding but the micro just started, branched out and through the teacup Yorkie genealogy.
Teacups are 6-4 pounds, where a micro is around 3 pounds. and again, please make sure you do not buy a runt! Runt do tend to have alot of health problems, and sometimes can catch up in growth any how.
There are tests they can run to see if the puppy is healthy and not a runt. Any professional breeder would be able to tell just from raising the puppy, and again refer to the parents.
Micros, being fairly new (under 20 generations) are not recognized, and so the breeder should state by the puppy the generation, such as F-3 meaning third generation in size.