Nico was not by any means the biggest player coming into his first season, 2 inches taller but more or less the same weight as Hughes and it was a point of contention before he played his first game as to how he would handle the league having a smaller sized build at the time and he handled it better than expected.
Zacha has a bigger build, but until the back half of this season, never consistently played as if he had that size. Nico and Bratt, who still both have far smaller builds to this day, have played bigger than Zacha ever has, even when Zacha was playing up to his potential and size in that time period this season. Heck, Brian Boyle, one of the biggest players in the league, who is an effective player in his role doesn't play as if he is his size and a number of smaller players throughout the league play as if they are his size.
It has to do with a players skating strength, skating ability, their vision and to another extent their confidence, not necessarily their size. Size helps, but is never the overriding factor. If Hughes is strong enough and confident enough at this point and is able to put himself in the correct positions in situations on the ice where he will not get outworked, he should be fine with his build currently and that bodes even better for him because of the amount of size he is capable of adding at his age.