Rift's dynamic content attempts to scale mob difficulty to the number of players present. However, even beyond the relatively simple events of the game's earliest zones, I'm finding that things are simply easier with more people.
I'm wondering if part of the problem is passive DPS healing. Mobs can and do get dramatically more health if they spawn in a crowded area, to account for the larger crowds beating on them. Though they do also get a damage bonus, there's an upper limit on this effect, because the health of any individual player in the group does NOT scale with the numbers present - if the mob does 20 times more damage when too many players show up, they'll start killing people in a single hit.
Meanwhile, Rift has several types of theoretically DPS characters who also generate passive healing as they attack. As more and more players show up, it seems like the raid gets more and more healing just because people have built this capability into their solo builds. As I've leveled through the zone events in Gloamwood and Scarlet Gorge, I'm noticing that even big AOE attacks don't do much to the raid's health meters because they're immediately topped off by a flurry of small green numbers.
In principle, it is good that zone bosses can take a while to kill - players who were on the other end of the zone to complete invasion objectives when the boss spawned should have time to reach the final showdown before the boss dies. I'm just saying that spending 10-15 minutes DPS'ing a boss who doesn't appear to be able to actually threaten any players can get old.