Most will continue to grow up to a year old. But some will still mature beyond that -- fill out more and gain in weight although not necessarily in size. Ben is 4 years old and was just recently neutered. He gained a 1/2 pound in the first 2 weeks after surgery and I think maybe a little more since. But he is still basically the same size, just a little heftier. Dogs do vary a great deal in how they grow just as people do. Some have spurts later in life, some stop growing earlier. That is why the growth charts are just estimates. Finished size is one of the nice little surprises our yorkies have for us.
Speaking of charts (
http://yorkiepassion.com/growthchart.htm) this one has a 26 week old weight of 53 ounces coming in as a 4 pound 18 month old. That is close to your situation so I think you can expect a little more growth but maybe not quite as much as you were thinking about.