He is CUTE!!!
But yes size is not an indicator at all. A Silky Terrier and a Yorkshire Terrier are different breeds. A Yorkie with a silky coat is just a description, but when you say Silky Terrier, you're talking bout a different breed lol. You could have a 7lb purebred Silky and a 15lb purebred Yorkie... it happens. Size is not the only indicator of breed type.
I did not get my Jackson from a good breeder (simply a classifieds ad out of the newspaper). I often think he is a Silky... he's longer than he is tall, he's 17lbs, his nose is longer, ears are bigger, and he honestly matches descriptions of Silky's more often than not. He doesn't really 'act' like most Yorkies I know. BUT... odds really are that he's a Yorkie. Just a badly bred one. Silky's are much more rare in the States. And puppy millers and bad breeders haven't got their hands on Silky's the way that they have for Yorkies which is why you get so many out of standard Yorkies that all look different. BUT, *some* 'breeders' (not good ones) do use Silky females to breed with Yorkie males as they do tend to be larger and throw more pups.
A wellbred Yorkie and a wellbred Silky are actually very easy to tell apart.
Here is a comparison of 3 similar terrier breeds, the Silky, the Yorkshire and the Australian. All closely related.
Comparison of Silky to Aussie & Yorkie
Here is some Silky champions:
Blackrange Championship Silky Terriers