If the dungeons are hard and long enough it's enough content. I remember vanilla Scholomance and only the best players able to clear it in quest mode (5 players) before Molten Core was fixed and on farm. Same to heroic Shattered Halls in Burning Crusade before the raidgear and nerfs made it easy. Size of the group does not matter if the challenge is big enough and developers do not chicken out and nerf it to the ground after a few weeks.