Swan, yorkie size is just a matter of preference. I have been raising yorkies for over 16 years and I just prefer yorkies in the 6 to 8 lb range. There are some breeders that prefer those under 5 lbs. I have never bred a female under 5 lbs. It is hard enough for the larger yorkies to have pups. I just can't imagine a poor little 3 to 4 lb yorkie having pups. It is to risky.
For show I believe the larger yorkies are just better movers and more elegant. For me the larger yorkies are just more breathtaking when they are in full coat.
Have you read this interesting article in the interpretation of the Yorkshire Standard by Carolyn Hensley?:
Young Yorkies Present AKC Standard Yorkshire Terriers Explained by Carolyn Hensley
I recommend anyone that breeds yorkies should read and understand the AKC standard for the Yorkshire Terrier.
I have been told by a number of show exhibitors that they go by the 3 strike rule when it comes to showing. That means if a dog has 3 or more faults then it should not be shown. Of course there are some faults that are so serious that it would only take one fault for the dog to be unshowable. Some faults that are borderline might be acceptable but some faults like a really bad front, rear or topline might prevent a dog from being shown. Also a Yorkie that is to shy would not be a good show prospect, even if it is near perfect structure wise.
Another thing to consider about size is that some yorkies continue to fill out and mature until they are 4 or 5 years old. You can have a beautiful bitch that weighs 6 to 7 lbs when she finishes her championship at age two but after maturing and having pups she could easily reach 8 or 9 lbs.
As for changing the Yorkshire Terrier standard, that would have to be done by the YTCA and from what I hear is that there is a lot of disagreement in the club over lots of issues. It's just like politics in our country, everyone has their own opinion on how things should be done and it is hard to get people to come together and agree on things.