Just my 2 cents on determining size. I will speak for myself and not say "all" but for me...I do not own a crystal ball (wish I did LOL) so I will never give a new yorkie parent a for sure size. All I can do is use the Yorkie Growth Chart, along with my knowledge of both mom and dads lines and give an educated GUESS.
I would never tell someone "Spike will be 3 lbs full grown, no less - no more " What I do say is "Spike is charting 3 lbs, or Spike is estimating to be 3 lbs give or take" etc...
The "Charts" are just that....a chart to "estimate" your dogs size. Each Yorkie line is different. Some grow fast and stop....some grow slow and speed up. ALOT more plays into the growth of a Yorkie than just their birth weight, their current weight etc.... Genetics can just not be predicted.
I get so many people wanting a 2 lb Yorkie. I don't breed for that size. Yes a tiny will happen, but I believe the smallest I have ever produced is 3 lbs. I prefer a nice fat healthy baby
Which goes to show, some of my babies do start out nice chubs, but tend to slow and stop growing.
I will only give an "educated guess". And a guess is all it is