Sorry, I'm not arguing that he isn't relatively slight compared to seasoned NHLers (he is), you just said he was slight based on the ratio of height/weight. I'm only noting that it shouldn't be a concern in the
slightest since he's a 22-yo rookie who hasn't had a full year of NHL strength and conditioning.
Check this article about a young Pronger, who was taller and weighed less than Muk at the draft. For reference, in 1996, at age 21/22, he was 6'6 200 (according to his official weight).
Pronger is choice for defense