Willander is 6'1, 190lbs as an 18 year old, Hronek is 6 feet, 190 pounds, and Hunter is 6 feet, 190 points. That's not undersized, size only comes in handy for stick checking (Carson Soucy) or clearing the crease (which is a bad position to be in if you're constantly defending your net).
You need mobile, transitional defenceman that can retrieve the puck fast and break out of the defensive zone, that's the ideal defenceman. You also don't have to be 6'4 to win board battles, Hughes and Garland excel that at by their skating and elusiveness.