I'm so glad Buddy found a good home, 7 pounds and under is standard, but many full blooded Yorkies weigh more than that. We have a club here at Yorkietalk called the Teapot Club, for those Yorkies who weigh more than 7 pounds, and it's one of the largest clubs! Bigger yorkies are better with children, and less likely to suffer from hypoglycemia. Here's the link if you're interested in joining the club.
http://www.yorkietalk.com/forums/group.php?groupid=5
Since the testicles make the bulk of testosterone, neutering helps with humping, but the adrenal glands, in both males and females, also make testosterone, and this varies quite a bit among dogs, even some female dogs that have been spayed hump. As long as your dog isn't humping a person, or the behavior is excessive, I wouldn't worry about it. Joey did this as a puppy, but about one month, after being neutered, he stopped, I guess his adrenal glands don't make that much.