Yorkies come in all sizes...depending on their line....A pups size isn't based on just its parents size...AKC standard is 4 lbs but not to exceed 7 lbs, meaning they want them to weigh in between 4-7 lbs for showing and a bitch must weigh over 5 lbs to breed....but a yorkies size goes much further back than just its parents...you could research many generation back in both parents line and find that a few pups full grown weighed in at 9 lbs but for the most part all were standard...so there is still a chance that even tho most were standard, that you could get a pup when full grown will be 8 to 9 lbs or more...Not even a show breeder who does research many generations back for size and health on both parents, can guarantee the size of their pups when full grown...and a yorkie will continue to grow and full out for the first 2 years...and their coat changes so much in the first 2 years, that theres really no guarantee of color or if they will have a silky or cotton coat.
And yes a show breeder that breeders to standards, can still have a tiny in the litter...hope this help.
and for the record I'm not a breeder...but I'm very good friends with my breeder and before I moved to South Fla, we lived only a few houses down from each other....I paid attention and learned alot from her.