What is the weight difference between a toy Yorkshire terrier and a teacup Yorkshire terrier?

there is no breed of yorkie called teacup....its just a runt... the breed size for a yorkie is about 7 lbs and up...if you are selling yorkies and someone insist on a teacup do not sell to them.... they are looking to breed runts to produce a runt litter......big mistake because the ancestors of the sire or bitch may be a much larger dog and your little runt may not be able to handle her pregnancy and die or have problems with delivery

Answer addition: Well, actually the breed standard for yorkies is less than 7 pounds. Therefore, the entire breed is called a "toy" breed. Some people consider yorkies to be teacups if they are under 4 pounds but that is not recognized by any breed standard at all. AKC does not recognize a difference and the previous answer was correct - the smaller the dog, the more difficult it is for them to survive giving birth. Plus, most smaller yorkies - 2-3 lbs are lucky if they give birth to even one pup in a litter!