We got ours from the shelter at 10.5 weeks and she was 16 pounds then. The vet said the shelter's age estimate was definitely within 2 weeks and very likely within one week of her true age. Last week she was 6 months old and weighed 41 pounds. She looks completely GSD, but it's likely she has something else in there too since we got her at the shelter.
My pup just turned 9 weeks and he weighs 20lbs. By his growth rate so far he'll probably be around 22 at 10 weeks.
Just like people, all GSDs grow at different rates and will end up different sizes. There is a range, and sounds like yours is still in the healthy range. (as long as ribs aren't protruding)
I've seen a male 110lbs gsd who was obviously obese as he wasnt tall enough with big enough bone structure to handle that weight. But just two days ago I saw another 110lbs GSD who was just straight beefy. Muscular, tall and very healthy, just a tank.