i disagree with your vet! he looks like a yorkie to me, and as it happens often he may just be on the bigger size. obviously your vet is not educated in small breeds, there is absolutely no poodle there hun. He might not meet the standard but he does look like a yorkie
just to confirm: when you bought your puppy did the breeder have any poodles? that would tell you if there was any possiblity, if she had only yorkies then there is no possibilty for a mix
and sometimes that have growth spurts when they are younger and stop growing, and in others they keep growing. my scrappy is the same size as my 6.5lb yorkie, but THICK, and has a LOT of thick heavy hair those to facts alone probably add 2-3lbs on him.
and he stopped growing at 5 months