Have you seen Bjorkstrands? Or when random people see Panarin walking down the street do you believe they think "damn, that dude definitely plays hockey". Physiques and builds get pretty overrated these days, you dont have to be a giant to have the strength and you dont need to be the strongest guy on the ice to be successful. Just like any 20 year old ever he will continue to get stronger with time.
As for his on ice play the coaches and management will decide, clearly they see enough to atleast give him a chance to prove himself. Hes mature enough to accept his AHL assignments fine so its easy enough to send him down and let him grow