Quoting: gk1991
Those are forwards. How many top pairing d-men out are 5-9 170 lbs. Torey Krug maybe but he's paired with Chara most of his career. Size does matter on defense.
Hughes is listed at 5'10 - 174 lbs. Here are notable defensemen around the same size. Keep in mind, that Hughes will also have roughly 1-2 years to bulk up a little bit.
Tyson Barrie is 5'10 - 190 lbs.
Ryan Ellis is 5'10 - 181 lbs.
Tobias Enstrom 5'10 - 181 lbs.
Jared Spurgeon 5'9 - 168 lbs.
Torey Krug 5'9 - 185 lbs.
Kris Russell - 5'10 - 172 lbs.
Sami Vatanen 5'10 - 187 lbs.
Shane Gostisbehere 5'11 - 181 lbs.
All of these guys are currently in the midst of very successful careers. By the time Hughes hits the NHL, he is going to be roughly the same size as these guys. He's gotta bulk up another 8-10 pounds and then he's right in line with these guys. The NHL is becoming a very speed friendly game. Hughes is the epitome of where the NHL is now. He isn't coming straight to the NHL this season. He still has another season or two at Michigan before he makes the jump. He's going to be a fantastic defenseman in the NHL, and if Detroit passes up on him because he might be a little smaller than most defensemen then that's going to be a decision that they could very well regret when Hughes makes it to the NHL.