You think Hughes will stay at 161lbs? He'll likely get above 170, and maybe even above 180 in the next year or so, which is fine. Alex DeBrincat, Johnny Gaudreau, Brayden Point, Artemi Panarin, Nikolaj Ehlers, Nico Hischier, Mitch Marner, Sebastian Aho, Patrick Kane, Nikita Kucherov etc. seem to be doing okay at that size or smaller...
Also, size has nothing to do with "concussion problems" in the NHL... Some of the most prominent names that have had their careers cut short due to concussion issues include Eric Lindros, Scott Steven, Keith Primeau, Johan Franzen, etc... all big men. Of course some smaller guys, namely Paul Kariya and Pat Lafontaine had their careers cut short due to concussions as well. Point is, there's no correlation between size and risk of concussions.
Dylan Cozens and Kirby Dach, who are both NHL size now, have just as much a risk of concussion as Jack Hughes. Maybe even more so because of how smart and elusive Hughes is on the ice.