Pastrnak was drafted at 168lbs. He went straight to the AHL then put up 27 points in 46 games for the Bruins. He now weighs 181.
Ehlers was drafted 165lbs. He played one more year of junior then joined the Jets, putting up 38 points in 72 games. He now weighs 172.
Dahlen is listed at 181 and is nearly 2 years younger. Not saying he will follow in their footsteps, but not every player has to bulk up to 190+ to play in the NHL. We don't know who can make the game at those lighter weights. Scouting report is that Dahlen is "stocky", with his weight in his lower body which should make him difficult to knock off the puck. Will depend on what mgmt deems best, but odds are we see him start in the AHL next year.