First of all, BIG yorkies do happen. Even with small siblings. So it could just be one of those things. Second, could be a mix. Can't momma's be pregant to more than one fella at the same time (will need a breeder to verify that), third, could be your breeders aren't being entirely truthful...who knows????
17 lbs is not a tiny yorkie, but it's still a little dog. I hope you love your baby regardless (please don't take offense, none is intended) and please, join our teapot club! (do a search on teapot on this site, you'll find TONS of pics of plus size yorkies)