If I might add my two cents worth here I must tell you that my oldest yorkie at this time is Sara. Now Sara is 6 yrs old and retired and the love of our lives ( along with all the rest LOL ) When Sara was having babies she had litters ( 5 in all ) of any where from 4 to 7 live freely whelped puppies. How lucky was that ?
My point here is that Sara weighs on a slim day about 7 1/2 lbs and every one of those babies were less than 4 lbs as adults. The daddy's weights were all in the 4 1/2 to 5 1/2 lb range. Several of her puppies stayed in that terribly tiny range of under 2 to 3 lbs. They were all healthy and lord knows I was never breeding to get tiny ones. I wanted another like Sara ( a champion ) but never got one large enough to show like her. So to answer the question that was raised of "where do the tinies come from"?
I would say there must be something in the genes behind that comes out. I guess I will never know about Sara as she never had a female I considered big enough to breed.
I think just love whatever you have and there is always going to be things that other people do or say that will annoy us but most of us need to focus on the good and not let annoyance rule our day. Be glad they love Yorkies and want to give it a home and you know most of them will get so attached that the weight issue will go away. JMHO