Lidstrom, Zetterberg and Datsyuk both came into the league around 175 lbs. and got up to 190-195 eventually. It's doable. I don't know if Jarnkrok will, but it's silly to think that Jarnkrok is doomed to be small because he is currently.
I hope he is. That way we won't have LGW still lamenting his trade 10 years from now.