Ok, so it's really hard to split instances into 16-24 guys when you have no idea how many people are arriving and how long before they turn up. So, rather than put 10 people in an instance and make them fail from lack of players, you cram 30 people into an instance and make them lag-fail. Not cool.
What if, for the first 3 minutes of the quest, you get zoned into a waiting room with say 4-6 other players of your level. You wait for the other players to arrive, then after 3 minutes you all get allocated into the relevant quest instance with a perfect distribution of players. Leave the gates open for another 2 minutes for late-comers to be evenly distributed among the instances.
To entertain those with short attention spans, you can spawn some mobs for them to whack in the waiting room so they can keep getting collectables. Because everyone needs more collectables than scales.